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Information Posted on 6/07/06
Notes for Chapter 22 – The Nature of Light
Section 1 – What is Light?
While other types of waves, such as sound or water, require a medium to travel, light
does not. Light is an electromagnetic wave – a wave than can travel through empty
space or matter and consists of changing electric and magnetic fields. As Figure 1, p. 632
shows, the electric and magnetic fields are at right angles – or are perpendicular – to each
other. The fields are also perpendicular to the direction of the wave motion.
Electric fields surround every charged object, and pull oppositely charged objects
towards it and repel like-charged objects. A magnetic field surrounds every magnet.
Because of magnetic fields, paper clips and iron filings are pulled toward magnets.
An electromagnetic (EM) wave can be produced by the vibration of an electrically
charged particle. When the particle vibrates, the electric field around it also vibrates. As
the electric field starts vibrating, a vibrating magnetic field is created. It is the vibration
of an electric field and a magnetic field together that produces an EM wave that carries
energy released by the original vibration of the particle. The transfer of energy as
electromagnetic waves is radiation.
In the near vacuum of space, the speed of light is about 300,000 km/s. Light travels more
slowly in air, glass, and other types of matter. Although light travels quickly, it takes
about 8.3 minutes for the sun’s light to reach the Earth. It takes this much time because
the Earth is 150,000,000 km away from the sun. The EM waves from the sun are the
major source of energy on Earth, and serve as the means by which plants and animals
survive.
Section 2 – The Electromagnetic Spectrum
The entire range of electromagnetic (EM) waves is called the electromagnetic spectrum
(Fig. 1, p. 636). As can be seen from this diagram, the spectrum is divided into regions
according to the length of the waves. Also, there is no sharp division between one kind
of wave and the next. Let’s take a look at the seven main parts of the EM spectrum,
going from left to right on the diagram:
Radio waves: These EM waves cover a wide range, and have some of the longest
wavelengths (longer than 30 cm.) and lowest frequencies. They are used for broadcasting
radio signals (Fig. 2, p. 637) and television signals.
Microwaves: These EM waves have wavelengths between 1 mm and 30 cm.
Microwaves are sent and received by cell phones, radar, and between Earth and satellites
in orbit. And they are also created in microwave ovens to heat foot (Fig. 3, p. 638).
Infrared: These EM waves vary between 700 nanometers (nm) and 1 mm. We feel
infrared waves as heat on our skin on a sunny day. Almost all things give off infrared
waves, including people! Infrared waves are invisible (as are all EM waves except
visible light), but some devices can detect infrared waves and generate a picture (Fig. 5,
p. 639).
Visible Light: These EM waves cover a very narrow range on the spectrum that humans
can see. They have wavelengths between 400 nm and 700 nm. Some of the sun’s energy
that reaches Earth is visible light. This visible light is white light, and contains all
wavelengths of visible light combined. We see these different wavelengths of visible
light as different colors (Fig. 6, p. 640). The colors of the visible spectrum can be easily
remembered by the memory aid ROYGBV.
Ultraviolet: This type of EM wave is also produced by the sun, with wavelengths
varying between 60 nm and 400 nm. Ultraviolet (UV) light affects your body in both
good and bad ways. Too much exposure to UV light can cause sunburn, skin cancer,
wrinkles, and eye damage. On the positive side, small amounts of UV light are essential
for skin cells to obtain vitamin D.
X-rays: These EM waves have wavelengths between 0.001 nm and 60 nm. They can
pass through many materials, and are therefore useful in the medical field for example, to
examine broken bones (Fig. 9, p. 642). This penetrating ability also allows x-rays to be
used as security devices in airports and other public buildings.
Gamma rays: These EM waves are the most energetic of all, having wavelengths less
than 0.1 nm. They can penetrate most materials very easily, and are used widely in the
medical field and other areas. For example, gamma rays are used to kill cancerous
tumors and kill bacteria in foods.
Section 3 – Interactions of Light Waves
Reflection happens when light waves bounce off an object. When you look in a mirror,
you are seeing light reflected twice – first from you and them from the mirror. The law
of reflection states that the angle of incidence is equal to the angle of reflection. Look at
Figure 1, p. 644. The beam of light traveling toward the mirror is the incident beam. The
beam of light reflected off of the mirror is called the reflected beam. The dotted line
perpendicular to the mirror is the normal. The angles mentioned above are created
between this normal and the incident and reflected beams.
Since a mirror’s surface is smooth, light reflects off all points of the mirror at the same
angle – this is called regular reflection. A wall’s surface, however, is rough. Light
beams hit the wall’s surface and reflect at many different angles – this is called diffuse
reflection (Fig. 2, p. 645). This second type of reflection explains why you can’t see your
reflection in a wall.
Objects that produce visible light such as the sun are called luminous. Visible objects
that are not light sources, but rather reflect light that strikes them, are called illuminated.
The moon and various objects around you are illuminated objects.
The transfer of energy by light waves to particles of matter is called absorption. A beam
of light shining through the air becomes dimmer the farther it travels because it is
absorbed by particles. Scattering is an interaction of light with matter that causes light
to change direction. Scattering is what makes the sky blue – shorter wavelength light is
scattered more than longer wavelength light.
Refraction is the bending of a wave as it passes at an angle from one substance to
another. Refraction of light happens because the speed of light varies as it travels from
one substance, or medium, to another. Light travels 300,000 km/s in a vacuum, but will
travel more slowly through air or other mediums such as glass. We can experience
refraction in a number of ways; for example, a straw in a glass of water appears bent
because light is moving from the medium of air to water (see also Fig. 6, p. 648). White
light passing through a prism can be separated into its component colors because of
refraction also (Fig. 7, p. 648).
Diffraction is the bending of waves around barriers or through openings. Light waves
can’t diffract much around large obstacles such as buildings (which is why we can’t see
around corners). It must pass through a narrow opening, around sharp edges, or through
a small barrier (Fig. 8, p. 649).
Interference is a wave interaction that happens when two or more waves overlap. When
two waves combine by constructive interference, the resulting wave has a greater
amplitude, or height, than the individual waves had. When waves combine by destructive
interference, the resulting wave has a smaller amplitude than the individual waves had.
Diffracted light waves can cause both types of interference (Fig. 9, p. 650).
Section 4 – Light and Color
When light strikes any form of matter, it can interact with the matter in three different
ways-it can be reflected, absorbed, or transmitted. Transmission is the passing of light
through matter. Look at Figure 1, p. 652. When you look out of a window on a sunny
day, you see objects beyond because light is transmitted through the glass. You can see
your reflection because light is reflected off the glass. Finally, the glass feels warm to the
touch because some light is absorbed by the glass.
Matter such as air, glass, and water through which visible light is easily transmitted is
transparent. Matter such as wax paper that transmits light but also scatters light is said
to be translucent. Matter that does not transmit any light is said to be opaque. You
can’t see through opaque objects such as things made of metal and wood.
We nonetheless see opaque objects because some of the light falling on them is absorbed,
and some is reflected. Only the reflected light reaches your eyes and it detected. A red
rose absorbs all colors except red. It is red wavelengths of light that are reflected off of
the rose and into your eyes, and you see the rose as being red. Objects appearing white
have all colors reflected, and objects appearing black have all colors absorbed.
The primary colors of light are red, green, and blue. When colors of light combine, you
see different colors. This is called color addition. When two colors of light are added
together, you see a secondary color of light. These secondary colors of light are cyan,
magenta, and yellow. See Figure 5, p. 655 for how these secondary colors of light are
formed. The colors on a television screen are produced by color addition of the primary
colors of light.
The situation changes when mixing colors of pigment. A pigment is a material that gives
a substance its color by absorbing some colors of light and reflecting others. Chlorophyll
is the pigment that gives plants their green color, and melanin is the pigment that gives
your skin its color. When you mix pigments together, more colors of light are absorbed
or taken away. So mixing colors of pigment is called color subtraction. The primary
pigments are yellow, cyan, and magenta. Secondary pigments are green, red, and blue.
Figure 6, p. 656 shows how the pigments combine to produce many different colors.
Information Posted on 5/15/06
Notes for Chapter 20 – The Energy of Waves
Section 1 – The Nature of Waves
A wave is any disturbance that transmits energy through matter or empty space.
Examples of waves include light waves, microwaves, sound waves, and water waves.
Energy can be carried away from its source by a wave: just drop a rock in a pond – waves
from the rock’s splash carry energy away from the splash.
However, it is important to understand that the material through which the wave travels
does not move with the energy – see Fig. 1, p. 574. As a wave travels, it does work on
everything in its path. For example, waves in a pond do work on anything floating on the
water’s surface – this is why leaves and other objects move up and down. And it is this
fact that things move on the water’s surface that tells you that the waves are transferring
energy.
Most waves transfer energy by the vibration of particles in a medium – a substance
through which a wave can travel. A medium (plural media) can be a solid, liquid, or
gas. When a particle vibrates (moves back and forth, as shown in Fig. 2, p. 575), it can
pass its energy to a particle next to it. The second particle will vibrate like the first
particle does. In this way, energy is transmitted through a medium.
Waves that need a medium are called mechanical waves. Sound waves and water waves
are examples of mechanical waves. Electromagnetic waves, however, do not need a
medium to transfer energy through, and can even go through matter, such as air, water,
and glass. For example, energy that reaches us from the sun goes through the vacuum of
space. Besides visible light, other types of electromagnetic waves include the other parts
of the electromagnetic spectrum: gamma rays, X-rays, ultraviolet light, infrared,
microwaves, and radio waves.
All waves transfer energy by repeated vibrations. However, waves can differ in many
ways. They can be classified based on the direction in which the particles of the medium
vibrate compared with the direction in which the waves move. The two main types of
waves are transverse waves and longitudinal waves. Transverse waves are waves in
which the particles of the medium move in an up-and-down motion (like the points on the
rope in Fig. 5, p. 577). Longitudinal waves are waves in which the particles of the
medium vibrate parallel to the direction of wave motion (like a spring pushed back and
forth as shown in Fig. 6, p. 578). When one end of the spring is pushed on, and coils
crowd together, and compression waves are created. When the spring is pulled back,
rarefaction waves are created. Sound waves are also longitudinal waves (Fig. 7).
When waves form at or near the boundary between two media, a transverse and
longitudinal wave can combine to form a surface wave. Here, the particles of the
medium move in circles rather than up and down.
Section 2 – Properties of Waves
By examining properties of waves, such as their height, waves can be compared and
described. The amplitude of a wave is related to its height. A wave’s amplitude is the
maximum distance that the particles of a medium vibrate from their rest position (Fig. 1,
p. 580). As demonstrated in class, shaking one end of a rope creates taller waves (larger
wave amplitude). A wave with a larger amplitude also carries more energy than one with
a smaller amplitude does.
Another property of waves is wavelength – the distance between any two crests or
compressions next to each other in a wave. The distance between two troughs or
rarefactions next to each other is also a wavelength. A wave with a shorter wavelength
carries more energy than a longer wavelength does. This was also demonstrated in class
with the rope, because more energy was put into the rope when it was shaken rapidly,
creating waves with smaller wavelengths.
The number of waves produced in a given amount of time is the frequency of the wave
(Fig. 3, p. 582). Frequency is usually expressed a unit called hertz (Hz). One hertz
equals one wave per second (1 Hz = 1/s). Again, we can return to the example of the
rope to better understand frequency and its relationship with energy – when the rope was
shaken quickly – taking more energy, high-frequency waves were made. So higher
frequency waves carry more energy than lower frequency waves.
Wave speed is the speed at which a wave travels. Wave speed (v) can be calculated
using wavelength (, the Greek letter lambda), and frequency (f), by using the wave
equation:
v =  x f
Three of the basic properties of a wave are related to one another in the wave equation –
wave speed, frequency, and wavelength. If you know any two of the properties, you can
use the equation to find the other.
Frequency and wavelength are inversely related: the higher the frequency, the shorter the
wavelength, and the lower the frequency, the longer the wavelength. For example, a
sound wave traveling underwater at 1,440 m/s that has a frequency of 360 Hz will have a
wavelength of 4.0 m, but a sound wave with twice the frequency will have a wavelength
half as big, and so will be 2.0 m in length.
Section 3 – Wave Interactions
Reflection happens when a wave bounces back after hitting a barrier (Fig. 1, p. 584). All
waves – water, sound, and light waves, can be reflected. We can see objects thanks to
their interaction with light. Of course, not all waves are reflected when they hit a barrier.
It this was the case, then the world would appear white! Additionally, if all waves
reflected off of your sunglasses, then you wouldn’t see anything! Sometimes waves are
transmitted, or passed through, a substance.
Refraction is the bending of a wave as the wave passes from one medium to another at
an angle. For example, a light wave passing at an angle into a new medium such as water
is refracted because the speed of the wave changes in the water (Fig. 2, p. 585).
Refraction was demonstrated in class with the pencil placed in a beaker of water, where it
appeared bent. Sometimes, as in sunlight, colors are refracted by different amounts.
When this happens, the light is dispersed, or spread out, into its separate colors. This is
how rainbows occur – raindrops split the sun’s white light into its component colors.
Diffraction is the bending of waves around a barrier or through an opening. If the barrier
or opening is larger than the incoming wave’s wavelength, there is only a small amount
of diffraction. If the barrier or opening is same size or smaller than the incoming wave’s
wavelength, the amount of diffraction is large (Fig. 3, p. 586).
The result of two or more waves overlapping and forming a single wave is called
interference. Depending on how the waves interact with each other results in either
constructive or destructive interference (Fig. 4, p. 587). When the crests of one wave
overlap the crests of another wave or waves (the troughs also overlapping) the result is
constructive interference. When waves combine in this way, the energy carried by the
waves is also able to combine – so the resulting wave has a larger amplitude than the
original waves had. Destructive interference happens when the crests of one wave and
the troughs of another wave overlap. The new wave has a smaller amplitude than the
original waves had.
In a standing wave (Fig. 5, p. 588) certain parts of the wave are always at the rest
position because of total destructive interference between all the waves. Other parts have
a larger amplitude because of constructive interference. The frequencies at which
standing waves are made are called resonant frequencies. When an object vibrating at or
near the resonant frequency of a second object causes the second object to vibrate,
resonance occurs. A resonating object absorbs energy from the vibrating object and
vibrates as well (Fig. 6, p. 588).
Information Posted on 5/5/06
Notes for Chapter 18 – Electromagnetism
Section 1 – Magnets and Magnetism
A magnet is any material that attracts iron or things made of iron. Magnets have three
properties: a north and south magnetic pole, magnetic force, and a magnetic field.
Each end of a magnet is a magnetic pole (north and south). Magnetic poles are points on
a magnet that have opposite magnetic qualities. Magnetic poles are always found in pairs
– no magnets that have only a north pole, or only a south pole, exist. In fact, cutting a
magnet in half will produce two new magnets, each with a north and south pole (Fig. 7, p.
514). One end of a magnet always points to the north - this is shown in a compass, which
contains a freely rotating magnet (Fig. 2, p. 511).
When two magnets are brought close together, the magnets each exert a magnetic force
on the other, due to the spinning electric charges in the magnets. The magnetic force
between magnets depends on how the poles of the magnets line up – like poles repel, and
opposite poles attract (Fig. 3, p. 511).
A magnetic field exists in the region around a magnet in which magnetic forces can act.
The shape of a magnetic field can be shown with lines drawn from the magnet’s north
pole to its south pole (Fig. 4, p. 512). These lines that map out the magnetic field are
called magnetic field lines. The closer the lines are together, the stronger the magnetic
field is. Since the magnetic field on a magnet is strongest at the poles, it is observed that
the lines around a magnet are closets together at the poles.
Whether or not a material is magnetic or not depends on its atoms. As electrons move
around, it makes, or induces, a magnetic field. This gives the atom a north and south
pole. In non-magnetic materials such as copper and aluminum, the magnetic fields of the
individual atoms cancel each other out. But in materials such as iron, nickel, and cobalt,
groups of atoms are in tiny areas called domains. The atom’s north and south poles in a
domain line up and make a strong magnetic field (Fig. 5, p. 513).
Dropping a magnet or hitting it too hard can move the domains, and the magnet can
become demagnetized. Another thing that can demagnetize a magnet is high temperature
(the atoms in a hotter material vibrate faster, and this causes the domains to no longer line
up, causing a loss of magnetism). It is possible to make a magnet, as Fig. 6, p. 513 shows
- an iron nail is magnetized by rubbing it in one direction with one pole of a magnet. The
domains in the nail become aligned, and it’s now capable of picking up the paper clip.
Some magnets are made of iron, cobalt, or nickel (or a mixture of those metals). These
type of magnets, called ferromagnets, have strong magnetic properties (Fig. 8, p. 514).
Another kind of magnet is an electromagnet, a magnet made by an electric current.
Magnets can also be described as temporary or permanent. Temporary magnets are made
of materials that are easy to magnetize, but lose their magnetization easily. Permanent
magnets are difficult to magnetize, but once they are, they tend to keep their magnetic
properties for a long period of time.
The Earth itself is one giant magnet, and in fact behaves as if it had a giant bar magnet
running down its center. The poles of this imaginary magnet are located near Earth’s
geographic poles (Fig. 9, p. 515). A compass needle points north because the magnetic
pole of Earth that is closest to the geographic North Pole is a magnetic south pole. So the
needle points to the north because its north pole is attracted to a very large magnetic
south pole.
What generates the Earth’s magnetic field is the movement of electric charges in the
Earth’s core, which is made of mostly of iron and nickel. The rotation of the Earth
causes the liquid in the core to flow. Electric charges move, and a magnetic field is
made. It is this magnetic field that helps to create the colorful aurora (Fig. 10, p. 516).
Section 2 – Magnetism from Electricity
Danish physicist Hans Christian Oersted discovered the relationship between electricity
and magnetism in 1820. From his experiments, he concluded that an electric current
produces a magnetic field. His work, along with that of other scientists, gave us insights
into electromagnetism – the interaction between electricity and magnetism.
The magnetic fields generated by the electric wires in Oersted’s experiments were not
strong enough to be very useful. However, a magnetic field can be strengthened with the
use of two devices - a solenoid and an electromagnet. A solenoid is a coil of wire that
produces a magnetic field when carrying an electric current (Fig. 2, p. 519). An
electromagnet is made up of a solenoid wrapped around an iron core. The magnetic
field of an electromagnet can be hundreds of times stronger than that of a solenoid.
These devices can be used in practical ways, as Figures 3 & 4 show, p. 520-521.
Just as a current-carrying wire causes a magnet to move, a magnet can also cause a
current-carrying wire to move (Fig. 5, p. 521). This property is useful in electric motors
– devices that change electrical energy into mechanical energy.
A galvanometer uses an electromagnet to measure electric current, and is sometimes
found in equipment used by electricians, such as ammeters and voltmeters (Fig. 7, p.
523).
Section 3 – Electricity from Magnetism
Just as a magnetic field can be generated from an electric current, so can an electric
current be generated from a magnetic field. Joseph Henry and Michael Faraday
conducted experiments that proved this (Figs. 1 & 2, p. 524-525). The process that they
discovered, in which an electric current is made by changing a magnetic field, is called
electromagnetic induction. As these two figures show, the current produced can be
increased if the magnet is moved through the wire coil at a faster rate – the faster
movement of the magnet causes its magnetic field to change faster, thereby inducing, or
creating, a greater electric current.
So Faraday’s experiments showed that an electric current is made when a magnet moves
in a coil of wire or when a wire moves between the poles of a magnet (Fig. 3, p. 526).
An electric generator uses electromagnetic induction to change mechanical energy into
electrical energy (Figs. 4 & 5, p. 526-527). The electric current produced by the
generator shown in Fig. 5 changes direction each time the coil make a half turn. Because
the current changes direction, it is an alternating current.
The energy that generators convert into electrical energy comes from different sources.
For example, in a nuclear power plant, it’s thermal energy from a nuclear reaction. The
energy boils water into steam, which turns a turbine. This turbine turns the magnet of the
generator, which makes an electric current and generates electricity.
Another device that uses the principle of electromagnetic induction is a transformer. A
transformer increases or decreases the voltage of alternating current. A simple
transformer is made up of two coils of wire wrapped around an iron ring. These two cold
are called primary and secondary. The number of loops in the primary and secondary
coils of a transformer determines whether it increases or decreases the voltage, as shown
in Fig. 7, p. 528). A step-up transformer increases voltage and decreases current. A stepdown transformer decreases voltage and increases current. The overall amount of energy
going into and out of the transformer, however, does not change.
The electric current that brings electricity into your home is usually transformed three
times (Fig. 8, p. 529). As the diagram shows, the power plant steps up the voltage
thousands of times. It’s then stepped down at a local power distribution center, and
stepped down yet again at a transformer near your house.
Information Posted on 4/26/06
Chapter 17 – Introduction to Electricity
Section 1 – Electric Charge and Static Electricity
Charge is a physical property – an object can have a positive charge, a negative charge, or
no charge at all. Charge is most easily understood by learning how charged objects
interact with each other. Charged objects exert a force – either a push or pull – on other
charged objects. The law of electric charges states that like charges repel, or push away,
and opposite charges attract (Fig. 2, p. 474). Without the attraction between protons and
electrons in an atom, atoms would fly apart.
The force between charged objects is an electric force. The size of this force depends on
two things: the amount of charge on each object (the greater the charge, the greater the
electric force is), and the distance between charges (the closer the charges are, the greater
the electric force). Charged objects are affected by electric force because charged objects
have an electric field around them. An electric field is the region around a charged
object in which an electric force is exerted on another charged object. A charged object
in the electric field of another charged object is attracted or repelled by the electric force
acting on it.
There are three ways to charge an object-see Fig. 3, p. 476: (1) friction happens when
electrons are ‘wiped’ from one object to another, (2) charging by conduction happens
when electrons move from one object to another by direct contact, and (3) induction,
which happens when charges in an uncharged metal object are rearranged without direct
contact with a charged object.
Just as with energy and mass, charges are not created or destroyed when you charge
something by any of the above methods – the number of protons and electrons stays the
same; electrons are simply moved from one atom to another. Because of this, charges are
said to be conserved, just as mass and energy are in interactions. You can use an object
called an electroscope to see if something is charged (Fig. 4, p. 477).
Most materials are either conductors or insulators, based upon how easily charges move
in them. An electrical conductor is a material in which charges move easily (most
metals are good conductors). An electrical insulator is a material in which charges
cannot move easily (plastic, wood, and rubber are examples of insulators).
Static electricity is the electric charge at rest on an object. The charges of static
electricity do not move away from the object that they are in – so the object keeps its
charge. The loss of static electricity as charges move off an object is called electric
discharge.
Lightning is another example of electrical discharge, and one that poses a potential
danger. A lightning rod is often mounted atop a building. Since it is joined to Earth by a
conductor, such as a wire, it is said to be grounded, providing a path for electric charges
to move to Earth.
Section 2 – Electric Current and Electrical Energy
Electrical energy is the energy of electric charges. In most of the things using electrical
energy, the charges flow through wires. This flow of charges is called electric current.
Electric current is the rate at which charges pass a given point. The higher the current
is, the greater the number of charges that pass the point each second. Electrical current is
expressed in units of amperes (often shortened to amps). When used in equations, the
symbol fir current is the letter I.
When a switch is flipped (as on a flashlight), an electric field is set up in the wire at the
speed of light. The electric field causes the free electrons in the wire to move. The
energy of each electron is transferred instantly to the next electron (Fig. 1, p. 482). You
can think if the electric field as a command to the electrons to charge ahead.
There are two kinds of electric current – direct current (DC), and alternating current
(AC). As Fig. 2, p. 483 shows, in direct current, the charges flow in the same direction,
whereas in alternating current, the charges continually shift from flowing in one direction
to flowing in the reverse direction. The electric current from outlets in your home is AC.
The electric current from the batteries found in a camera, for example, is DC.
Voltage is defined as the potential difference between two points in a circuit, and is
expressed in volts (V). Here is a way to envision how voltage works: imagine that you’re
on a bike at the top of a hill. You can roll down the hill because of the difference in
height between the two points. The “hill” that causes charges in a circuit to move is
voltage. Voltage is a measure of how much work is needed to move a charge between
two points. You can think of voltage as the amount of energy released as a charge moves
between two points in the path of a current. The higher the voltage is, the more energy is
released per charge.
Resistance is another factor that determines the amount of current in a wire. Resistance
is defined as the opposition to the flow of an electric charge, and is expressed in ohms.
When used in equations, the symbol for resistance is the letter R. You can think of
resistance as “electrical friction” – the higher the resistance of a material is, the lower the
current in the material. Good conductors such as copper have low resistance. Poor
conductors, such as iron, have higher resistance. The resistance in insulators such as
plastic and rubber is so high that electric charges cannot flow in them.
The thickness and length of a wire also affects its resistance, as shown in the model in
Fig. 5, p. 486. A thick pipe allows more electric charges to pass through. A short pipe
has less resistance than a long pipe does because the charges do not have to travel as far.
Resistance also depends on temperature. The atoms of a material vibrate faster at a high
temperature, and get in the way of the flowing electric charges. So material at a high
temperature has high resistance, and material at a low temperature has low resistance.
Materials at a very low temperature are called superconductors, and very little energy is
wasted when electric charges move in them.
To generate electrical energy, cells change chemical or radiant energy into electrical
energy. Batteries are made of one or more cells. As Fig. 7, p. 487 shows, cells can
contain a mixture of chemicals called an electrolyte, which allows charges to flow. Cells
also contain electrodes made from conducting materials. The electrode is the part of the
cell where charges enter or exit. There are two kinds of cells; wet and dry cells. A car
battery is made of several wet cells that use sulfuric acid as the electrolyte. The
electrolytes in dry cells are solid or paste-like. Small radios and flashlights use dry cells.
Thermocouples convert thermal energy into electrical energy. Joining wires of two
different metals into a loop makes a simple thermocouple, like the one shown in Fig. 9, p.
488. The temperature difference within the loop causes charges to flow through the loop
– the greater the temperature difference, the greater the current. Photocells convert light
energy into electrical energy. When light shines on a photocell, electrons in the silicon
atoms gain enough energy to move between atoms. This energy is then able to move
through a wire to provide electrical energy.
Section 3 – Electrical Calculations
Georg Ohm, a German school teacher, was instrumental in discovering how current,
voltage, and resistance are related. In studying the resistance of materials, he measured
the current that resulted from different voltages applied to a piece of metal wire. As the
graph on the left of Fig. 1, p. 490 shows, Ohm found that the ratio of voltage (V) to
current (I) is a constant for each material. This ratio is the resistance (R) of the material.
When the voltage is expressed in volts (V) and the current is in amperes (A), the
resistance is in ohms (). The following equation is called Ohm’s law because of the
work that Ohm did:
V
R = ---------- , or V = I X R
I
As the graph on the right of Fig. 1 shows, as the resistance goes up, the current goes
down, and as the resistance decreases, the current increases.
The rate at which electrical energy is changed into other forms of energy is electric
power. The unit for power is the watt (W), and the symbol for power is the letter P.
Electrical power is calculated by using the following equation:
power = voltage x current, or P = V x I
Light bulbs have power rating measured in watts (typically 60 W, 75 W, or 120 W).
Another common unit of power is the kilowatt (kW). One kilowatt is equal to 1,000 W.
Kilowatts are typically used to express high values of power, such as that needed to heat
a house.
The amount of electrical energy used in a home depends on the power of the electrical
devices in the house and length of time that those devices are on. The equation for
determining electrical energy is:
electrical energy = power x time, or E = P x t
Different amounts of electrical energy are used each day in a home. Electric companies
usually calculate electrical energy by multiplying the power in kilowatts by the time in
hours. So the unit of electrical energy is usually kilowatt-hours (kWh). If 2,000 W of
power are used for 3 h, then 6 kWh of energy were used. Power companies use meters,
such as the one shown in Fig. 3, p. 492. You can reduce your electric bill by taking
practical steps, such as running a fan as much as possible (rather than the air conditioner,
and turning off lights when they are not in use.
Section 4 – Electric Circuits
Just like a roller coaster, an electric circuit always forms a loop – it begins and ends in the
same place. Because a circuit forms a loop, a circuit is a closed path. So, an electric
circuit is a complete, closed path through which electric charges flow. As Fig. 1, p. 494
illustrates, all circuits need 3 parts: an energy source, wires, and a load. Loads such as a
light bulb or radio are connected to the energy source by wires. The function of loads is
to change electrical energy into other forms of energy such as thermal, light, or
mechanical energy. As the load changes electrical energy into other forms, they offer
some resistance to electric circuits.
Sometimes a circuit contains a switch (Fig. 2, p. 495). The function of the switch is to
open and close a circuit, and is made of two pieces of conducting material, one of which
can be moved. In order for charges to flow through a circuit, the switch must be closed,
or ‘turned on.’ If a switch is open, or ‘off,’ the loop of the circuit is broken. Light
switches, power buttons on radios, and keys on computers open and close circuits.
A circuit can be a series circuit or a parallel circuit. One main difference between these
two types of circuits is the way in which the loads are connected. A series circuit is a
circuit in which all parts are connected in a single loop (Fig. 3, p. 496). There is only one
path for charges to follow, so charges moving through a series circuit must flow through
each part of the circuit. All of the loads in a series circuit share the same current. A
parallel circuit is a circuit in which loads are connected side by side. Charges in a
parallel circuit have more than one path on which they can travel. Unlike a series circuit,
the loads in a parallel circuit do not have the same current.
In homes, several circuits connect all of the lights, appliances, and outlets. The circuits
branch out from a breaker box or a fuse box that acts as “electrical headquarters” for the
building. Each branch receives a standard voltage, which is 120 V in the U.S.
In a circuit failure, broken wires or water cause a short circuit. In this case, charges do
not go through one or more loads in the circuit. The resistance decreases, so the current
increases. This leads to the wires heating up, causing the circuit to fail. Safety features
such as fuses and circuit breakers help to prevent electrical fires.
A fuse is a thin strip of metal; charges in the circuit flow through this strip. If the current
is too high, the metal strip can melt, as shown in Fig. 5, p. 498. As a result, the circuit is
broken, and charges stop flowing.
A circuit breaker is a switch that automatically opens if the current is too high. A strip of
metal in the breaker warms up, bends, and opens the switch, which opens the circuit,
stopping the flow of charges. Open circuit breakers can be closed by flipping a switch
after the problem has been fixed.
Information Posted on 4/6/06
Chapter 16 – Atomic Energy
Section 1 - Radioactivity
French scientist Henri Becquerel’s key experiment with fluorescent minerals was perhaps
the first insight into the nature of radioactivity. The mineral that he was working with
had given off energy that had passed through paper that was wrapped around the
photographic plate that he had placed the mineral on. Becquerel concluded that the
energy had come from uranium, an element contained in the mineral he was working
with. This energy that Becquerel observed is called nuclear radiation – high energy
particles and rays given off by the nuclei of some atoms.
Marie Curie, working with Becquerel, named the process by which some nuclei give off
nuclear radiation radioactivity, also called radioactive decay. During radioactive decay,
an unstable nucleus gives off particles and energy. There are three kinds of radioactive
decay: alpha decay, beta decay, and gamma decay.
Alpha decay is the release of an alpha particle from a nucleus. It has a mass number of 4
and a charge of 2+. Recall that the mass number is the sum of protons and neutrons in
the nucleus of a helium atom. So an alpha particle is in fact helium-4. Many large
radioactive nuclei give off alpha particles and become nuclei of different elements – see,
for example, how radium-226 decays into radon-222 and an alpha particle (Fig. 2, p.
449).
It is important to understand that mass and charge are always conserved in radioactive
decays. Just like in chemical equations, where the masses of the reactant and product
sides must be equivalent, the decay products must have the same mass (and charge) as the
nuclei did prior to decay. Look again at Figure 2: the mass numbers before and after the
decay are 226. Charge is also conserved – both before and after the decay, the charge is
88+.
Beta decay is the release of a beta particle from a nucleus. A beta particle can be either
an electron (with a charge of 1-), or a positron (with a charge of 1+). Again, as Figure 3,
p. 450 shows, both mass and charge are conserved. Also, notice that, as in alpha decay,
beta decay involves the original nucleus decaying into a nucleus of a different element.
There are two types of beta decay. This is because not all isotopes of an element decay
in the same way. Recall that isotopes are atoms that have the same number of protons as
other atoms of the same element do, but different numbers of neutrons. In one type of
beta decay, a neutron breaks into a proton and electron – this occurs when carbon-14
decays, shown in Fig. 3. In the second type, a proton breaks into a positron and a neutron
– this occurs when carbon-11 decays.
Gamma decay involves the release of gamma rays from a nucleus. Energy is also given
off during alpha decay and beta decay. Some of this energy is in the form of very high
energy light called gamma rays. Gamma rays – since they are light – have no mass or
charge. Therefore, gamma decay alone doesn’t cause one element to change into another
element – this can only happen with alpha or beta decay.
These three different forms of nuclear radiation have different abilities to penetrate, or
go through, matter. This different is due to their mass and charge, as Figure 4, p. 451
shows. Alpha particles, since they’re made up of 2 protons and 2 neutrons, have the most
mass and charge of any type of decay. Because of this, they are able to be stopped, or
absorbed, before penetrating far. Beta particles, being electrons or positrons, have a 1- or
1+ charge and almost no mass. Because of this, they are more penetrating than alpha
particles. Since gamma rays have no mass or charge, they are the most penetrating, and
can cause damage deep within matter. Alpha particles, although only slightly
penetrating, can cause the most damage since they’re the most massive.
Each radioactive isotope has its own rate of decay, called half-life. A half-life is the
amount of time it takes one-half of the nuclei of a radioactive isotope to decay. The table
on p. 453 shows the half-lives of several isotopes. As you can see, there can be a wide
range of half-lives: uranium-238 has a half-life of 4.5 billion years! Oxygen-21, on the
other hand, has a half-life of 3.4 seconds! Look at Figure 6 on p. 453 to see how the
process works – after one half-life, one-half of the original sample has decayed, and the
other half is unchanged. After two half-lives, only one fourth of the original sample now
remains unchanged. Remember, a half-life involves half of the sample of the isotope –
that’s why, after three half-lives, there’s only one-eighth of the original sample that
remains. Let’s look at an example: suppose we have 20 grams of nitrogen-13, which has
a half-life of 10 minutes. In 20 minutes, how much of the original sample will we have?
Well, after 10 minutes (one half-life), we’ll have half of the original sample – 10 grams.
In another 10 minutes (20 minutes total now), two half-lives have completed, and so we
have now half of 10 grams, or 5 grams – one-fourth of the original 20 gram sample.
Scientists can use some radioactive isotopes to provide accurate ages (dates) for a variety
of objects, from fossilized remains to the age of ancient meteorites.
Radioactivity, besides being used to determine the ages of different objects, has a number
of uses in healthcare and in industry. In your own home, your smoke detector may have
a small amount of radioactive americium. Radioactive materials can be used to treat
illnesses, including cancer. They can also be used in a number of industrial settings, such
as testing the thicknesses of metal sheets, and even to power space probes.
Section 2 – Energy from the Nucleus
There are ways that the energy of the nucleus can be harnessed for constructive - and
destructive – purposes. Nuclear fission is the process in which a large radioactive
nucleus splits into two small nuclei and releases energy. Figure 1, p. 456 shows the
decay of uranium-235.
Matter can be changed into energy, from Einstein’s famous equation E = mc2. What
Einstein showed was that matter is a form of energy. In Figure 1, if you compare the
total mass of the products (233) with the total mass of the reactants (235), you’d see that
the total mass of the products is slightly less than the total mass of the reactants. Why are
the masses different? Do we have a violation of the principle of the conservation of
mass? Not at all – some of the matter was converted into energy, in this case three
neutrons.
During nuclear fission, such as we have shown in Figure 1 with a uranium-235 nucleus,
the neutrons produced can split other uranium-235 nuclei, so that energy and more
neutrons are given off. This process can keep continuing, leading to a nuclear chain
reaction – a continuous series of nuclear fission reactions (see Fig. 3, p. 457). When
this process is uncontrolled, huge amounts of energy are given off very quickly – such as
in an atomic bomb. However, nuclear power plants (Fig. 4, p. 458) use controlled chain
reactions, and in this case large amounts of electricity can be generated that can provide
power to millions.
There are advantages and disadvantages to using fission as a form of energy. On the
down-side, there is the possibility of accidents occurring, such as the one that occurred in
Chernobyl, Ukraine, in 1986. The nuclear waste produced (this includes fuel rods,
chemicals used to process uranium, etc.) is also a serious problem – the waste will give
off high levels of radiation for thousands of years. On the up-side, while nuclear power
plants cost more to build than those that use fossil fuels like coal, nuclear power plants
often cost less to run. They also don’t release gases like carbon dioxide into the
atmosphere. However, the supply of uranium, like coal and other fossil fuels, is limited.
Fusion is another type of nuclear reaction in which matter is converted into energy. In
nuclear fusion, two or more nuclei that have small masses combine, or fuse, to form a
larger nucleus. This is the process that generates energy in the sun and other stars. In the
case of the sun and stars of similar mass, four hydrogen nuclei - the lightest element fuse together to make a helium-4 nucleus - the next lightest element (Fig. 6, p. 460).
In order for fusion to happen, the repulsion between positively charged nuclei must be
overcome. Recall that protons have positive charges, which will cause them to repel, or
push away from each other. Very high temperatures are needed to make the protons
come very close to each other, and fuse – more than 100,000,0000C! These high
temperatures cause matter to be in a state called plasma, when electrons have been
removed from atoms. So plasma is made up of ions and electrons.
Like fission, fusion has its disadvantages and advantages. On the down-side, very high
temperatures are needed. Additionally, more energy is needed to make and hold the
plasma than is generated by fusion. Scientists feel that these problems can be resolved,
perhaps within a few decades. On the up-side, fusion reactors are less accident prone
than fission reactors. Fusion reactors also release more energy per gram than fission
reactors do, and they are cleaner burning, producing less radioactive waste than fission
reactors.
Information Posted on 4/2/06
Chapter 15 – Chemical Compounds
Section 1 – Ionic and Covalent Compounds
One way of grouping compounds is by the kind of bonds that they have. A chemical
bond is the combining of atoms to form molecules or compounds. This bonding can
occur between the valence electrons (those in the outermost energy level) of different
atoms. This section focuses on two different compounds that can be formed, ionic and
covalent.
Compounds that contain ionic bonds are called ionic compounds. An ionic bond is an
attraction between oppositely charged ions. Ionic compounds can be formed by the
reaction of a metal with a nonmetal. Therefore, when ionic compounds form, there is a
transfer of electrons from the metal to the nonmetal. This transfer also causes atoms to
become ions. A good example of an ionic compound is sodium chloride (table salt).
When sodium (a metal) gives up its one valence electron to chlorine, sodium becomes a
positively charged ion, and chlorine, since it receives an electron, becomes a negatively
charged ion.
Properties of ionic compounds include brittleness – the tendency to break apart when hit.
This is due to the arrangement of ions in a crystal lattice - a repeating three-dimensional
pattern (see Fig. 1, p. 418). Ionic compounds, because of the strong ionic bonds that hold
them together, have high melting points. Sodium chloride, for example, melts at 801oC.
Fig. 2, p. 419 shows the melting points for other ionic compounds.
Many ionic
compounds are highly soluble – that is, they are easily dissolved in water. Water
molecules attract each of the ions in an ionic compound and pull them away from each
other. The resulting solution can conduct an electric current (Fig. 3, p. 419) because the
ions are charged. An undissolved crystal of an ionic compound does not conduct an
electric current.
Most compounds are covalent compounds – compounds that form when a group of
atoms share electrons. This sharing of electrons forms a covalent bond, and the atoms
that join together to form a covalent bond are nonmetals. The group of atoms that make
up a covalent compound is called a molecule – the smallest particle into which a
covalently bonded compound can be divided and still be the same compound.
The properties of covalent compounds are very different from those of ionic compounds.
Because the bonds that hold covalent compounds together are weaker, less heat is needed
to break them apart – therefore covalent compounds have a low melting point. Also,
many covalent compounds are not soluble in water – meaning that they don’t dissolve
easily in water (oil in salad dressing is a good example). Some covalent compounds do
dissolve in water (such as sugar). However, when sugar dissolves in water, ions are not
formed, therefore a solution of sugar and water does not conduct electricity, as shown in
Fig. 5, p. 421. Some covalent compounds, such as acids, do form ions when dissolved in
water, and will conduct an electric current.
Section 2 – Acids and Bases
An acid is any compound that increases the number of hydronium ions, H3O+, when
dissolved in water. Hydronium ions form when a hydrogen ion, H+, separates from the
acid and bonds with a water molecule, H2O, to form a hydronium ion, H3O+. Some of the
properties of acids include a sour flavor (such as in a lemon or lime). The taste of such
foods is a result of citric acid. Many acids are corrosive, meaning that they destroy tissue
and many other things. Acids change color in indicators. An indicator is a substance
that changes color in the presence of an acid. Some indicators include bromthymol blue
(Fig. 2, p. 423) and litmus, which contains strips of red and blue paper. When an acid is
added to blue litmus paper, the color of litmus changes to red. In our acids-bases lab,
citric acid turned the indicator paper a very deep shade of red.
Acids also react with some metals, producing hydrogen gas. For examples, hydrochloric
acid reacts with zinc metal to produce hydrogen gas (Fig. 3, p. 423). Acids also conduct
electric current. When acids dissolve in water, they break apart and form ions in the
solution, which make it possible for the solution to conduct an electric current. An
example of this is a car battery. Sulfuric acid in the battery conducts electricity to help
start the car’s engine. Acids, such as sulfuric acid, nitric acid, and hydrochloric acid, are
used in many areas of industry and in homes. They are used to make products including
paper, paint, detergents, fertilizers, rubber, and plastics.
swimming pools to help keep them algae-free.
They can even be used to in
A base is any compound that increases the number of hydroxide ions, OH-, when
dissolved in water (Fig. 5, p. 425). These hydroxide ions give bases their properties,
which include a bitter flavor (i.e., as tasting soap will show!), a slippery feel (as soap
feels). Many bases, like acids, are corrosive. Also like acids, bases change color in
indicators. Bases change the color of red litmus paper to blue. Or, if using the indicator
bromthymol blue, it turns a deep shade of blue when a base is added to it, as shown in
Fig. 6, p. 426. In our acids-bases lab, ammonia turned the indicator paper a deep blue.
Because bases, when dissolved in water, increase the number of hydroxide ions, OH-, an
electric current can be conducted in solutions of bases.
Like acids, bases have many uses. For example, sodium hydroxide is used to make soap
and paper. Calcium hydroxide is used to make cement and plaster, and ammonia is found
in many household cleaners. Magnesium hydroxide and aluminum hydroxide are used in
antacids to treat heartburn.
Section 3 – Solutions of Acids and Bases
Acids and bases can be strong or weak. This strength or weakness is not the same as the
concentration of an acid or base. The concentration is defined as the amount of acid or
base dissolved in water. The strength of the acid or base depends on the number of
molecules that break apart when the acid or base is dissolved in water.
As an acid dissolves in water, its molecules break apart and form hydrogen ions, H+. If
all of the acid’s molecules break apart, the acid is a strong acid. Examples of strong
acids include hydrochloric acid and sulfuric acid. If only a few molecules of the acid
break apart, then the acid is a weak acid. Weak acids include citric acid and carbonic
acid. In a similar manner, if all of the molecules of a base break apart in water to produce
hydroxide ions, OH-, then the base is a strong base. Strong bases include sodium
hydroxide and potassium hydroxide. When only a few of the base’s molecules break
apart, then it is a weak base, such as ammonium hydroxide and aluminum hydroxide.
When acids and bases meet and undergo a reaction, it is called a neutralization reaction.
Acids and bases neutralize each other – take away any ions – because the hydrogen ions
(H+), present in an acid, and the hydroxide ions (OH-), which are present in a base, react
to form water, H2O, which is neutral. The chemical equation is: H+ + OH-  H2O
Notice that the positive and negative charges cancel each other out, giving us neutral
water. In some cases, the water evaporates, and the ions join together to form a
compound called a salt.
As we have seen, an indicator, such as litmus, can be used to identify an acid or a base.
The pH scale is used to describe how acidic or basic a solution is. The pH of a solution
is a measure of the hydronium ion concentration in the solution. A pH of 7 is neutral –
neither acidic nor basic. Basic solutions have a pH greater than 7, and acidic solutions
have a pH less than 7 (see Fig. 2, p. 429). Besides litmus strips or hydrion paper, pH
meters can be used to detect and measure hydronium ion concentration electronically.
Living things depend on having a steady pH in their environment. For example, some
plants prefer acidic soil. Flowers of different colors can be produced by growing them in
soil that is acidic or basic (Fig. 4, p. 430).
When an acid neutralizes a base, a salt and water are produced. A salt is an ionic
compound that forms when a metal atom replaces the hydrogen of an acid. Salts have
many uses in industry and in homes. For example, sodium chloride is used to season
foods. Sodium nitrate is a salt used to preserve food, and calcium sulfate is used to make
wallboard, used in construction. Salt is also used to help keep roads free of ice by
decreasing the freezing point of water.
Section 4 – Organic Compounds
Organic compounds are covalent compounds composed of carbon-based molecules.
Fuel, rubbing alcohol, cotton, paper, plastic, and sugar are all examples of organic
compounds. All organic compounds contain carbon. Each carbon atom contains four
valence electrons. So each carbon atom can make four bonds with four other atoms.
Figure 1, p. 432 shows the three structural formulas for organic compounds. They are
used to show how atoms in a molecule are connected. They are also called carbon
backbones because carbon forms the basis for the three types of chains – straight,
branched, and ring.
Although many organic compounds contain several kinds of atoms, some contain only
two – hydrogen and carbon. These organic compounds containing only these elements
are called hydrocarbons, of which there are three groups (see Figs. 2 & 3, p. 433).
Saturated hydrocarbons, or alkanes, have each carbon atom in the molecule sharing a
bond with each of four other atoms. A single bond is a covalent bond made up of one
pair of shared electrons. An unsaturated hydrocarbon, such as ethane or ethyne, has at
least one pair of carbon atoms sharing a double or triple bond. They are called
unsaturated because the double or triple bonds can be broken and more atoms can be
added to the molecules. Hydrocarbons that contain two carbon atoms connected by a
double bond are called alkenes, and hydrocarbons that contain two carbon atoms
connected by a triple bond care called alkynes. The third type of hydrocarbon is the
aromatic hydrocarbons, most of which are based on benzene. As shown in Fig. 3,
benzene has a ring of six carbons that have alternating double and single bonds.
Organic compounds that are made by living things are called biochemicals, and are
divided into four categories: carbohydrates, lipids, proteins, and nucleic acids.
Carbohydrates are biochemicals such as cellulose and glycogen, and are composed of
one or more simple sugar molecules bonded together, and are used as a source of energy.
There are two kinds of simple carbohydrates: simple and complex. Lipids are
biochemicals that do not dissolve in water, and include fats, oils, and waxes. Proteins
are biochemicals that are composed of ‘building blocks’ called amino acids. Nucleic
acids are biochemicals made up of nucleotides, molecules made of carbon, hydrogen,
oxygen, nitrogen, and phosphorus atoms. Nucleic acids are sometimes called the
blueprints of life, because they contain all the information needed for a cell to make all of
its proteins. The two kinds of nucleic acids are DNA and RNA.
Notes for Quarter 3
Information Posted on 3/20/06
Chapter 14 – Chemical Reactions
Section 1 – Forming New Substances
A chemical reaction is a process in which one or more substances changes to make one
or more new substances. You know that a chemical reaction has occurred when the
chemical and physical properties of the new substances differ from those of the original
substances. A good example is photosynthesis, the process whereby plants make food
for themselves. Three substances chemically react with each other – sunlight, carbon
dioxide, and water, and produce new substances – glucose (sugar) and oxygen:
sunlight + H2O + CO2
glucose + O2
The above equation is properly read “sunlight reacting with water and carbon dioxide
produces glucose and oxygen.” The substances on the left side of the arrow are called the
reactants (since they react chemically with one another), and the substances on the right
side are called the products (since they are the substances produced as a result of the
chemical reaction).
There are several signs that you can observe that tell you that a chemical reaction has
taken place. In some chemical reactions, gas bubbles form. Others form solid
precipitates – solid substances that are formed in a solution. During other chemical
reactions, energy is given off, in the form of light, thermal energy (heat), or electricity.
Reactions often have more than one of these signs present (see Fig. 2, p. 389 for a
diagram showing these four common signs.
As already mentioned, the properties of substances that are formed in a chemical reaction
(the products) are very different from the substances that react (the reactants). Sodium is
a soft metal that reacts violently with water. Chlorine is a poisonous yellow-green gas.
But when they react together, a new substance – sodium chloride (table salt) – is
produced. This new substance has very different properties than sodium and chlorine
alone – table salt is a white, granular solid safe to put on our food.
In order for new substances to form in a chemical reaction, the chemical bonds in the
starting substances (the reactants) must break. This can be done when molecules collide
with enough energy. The atoms then rearrange, and new bonds form to make the new
substances, as Fig. 4, p. 390 shows in the example of hydrogen (H2) and chlorine (Cl2).
Section 2 – Chemical Formulas and Equations
All substances are formed from about 100 elements. Each element has a chemical
symbol. Chemical formulas are a shorthand way to use chemical symbols and numbers
to represent a substance. The chemical formula F2 means that this fluorine molecule is
made up of two fluorine atoms. The chemical formula for NH4 tells us that 1 atom of
nitrogen and 4 atoms of hydrogen bonded together to make the compound ammonia.
Look at Fig. 1, p. 392 for some other examples.
We can write formulas for both covalent compounds and ionic compounds. You can
write formulas for covalent compounds by using the prefixes in the names of the
compounds. For example, take carbon dioxide (CO2). Like the examples above, when
there is an absence of a prefix (like we have here for carbon), it indicates one atom. So
there is one carbon atom, and two oxygen atoms – but notice the prefix di- in front of
oxide – this prefix tells us that there are two atoms of oxygen. With dinitrogen
monoxide (N2O), the prefix di- indicates two nitrogen atoms, and the prefix monoindicates one oxygen atom (Table 1 on p. 393 gives you a list of commonly used
prefixes).
When we write formulas for ionic compounds, you have to be sure that the compound’s
charge is 0. Recall that ionic compounds are formed from a metal atom and a nonmetal
atom. So, the compound magnesium chloride (MgCl2) is formed from Mg and Cl2
bonded together. The magnesium ion has a 2+ charge (it’s a metal and gives up its 2
valence electrons when it reacts with chlorine). The chloride ions have a 1- charge (it’s a
nonmetal, and the two chlorine atoms will receive the two electrons from magnesium).
Add up the charges: one magnesium ion has a charge of 2+, and two chloride ions will
have a charge of 2- together – so we have a total charge of 0. And that’s why we write
the formula for magnesium chloride as MgCl2. Fig. 3, p. 393 gives you another example
using NaCl.
Chemical equations use chemical symbols and formulas as a shortcut to describe a
chemical reaction. As explained in the notes for section 1 using the example of
photosynthesis, the substances that start a chemical reaction are called the reactants, and
are listed to the left of the arrow. The substances that are formed from the reaction are the
products, and are listed to the right of the arrow. So in a chemical equation like the one
listed in Fig. 5, p. 394 (C + O2
CO2), what the equation is telling us is that one atom
of carbon is reacting with two atoms of oxygen to form the compound carbon dioxide.
Notice the plus sign (+) on the reactant side – this always separates the formulas of two
or more reactants or products from one another. The number 2 that you see is called a
subscript, and is always written below the element or formula symbol. The arrow (
),
also called the yields sign, separates the formulas of the reactants fro the formulas of the
products.
Notice the above equation: on the reactant side, there is one atom of carbon, and two
atoms of oxygen. On the product side, there is one atom of carbon, and two of oxygen.
There’s a balance; an equality of atoms on both the reactant and product side. So this
equation is said to be balanced. But why must chemical equations be balanced? IN the
1700’s, French chemist Lavoisier found that the total mass of the reactants was always
the same as the total mass of the products. His work led to the law of conservation of
mass, which states that mass is neither created nor destroyed in ordinary chemical and
physical changes. So the numbers and kinds of atoms on both sides of the arrow must be
equal – just like our above example. To balance equations, you use coefficients, which
are numbers placed in front of a chemical symbol or formula. For example, 2CO
represents two carbon monoxide molecules. The number 2 is the coefficient. For an
equation to be balanced, all atoms must be counted. So you must multiply the subscript
of each element in a formula by the formula’s coefficient. For example, 2H2O contains a
total of 4 hydrogen atoms and two oxygen atoms. Only coefficients – NOT
SUBSCRIPTS – are changed when balancing equations. Look at Figure 7, p. 396 for
a step-by-step example of how to balance the equation H2 + O2
H2O.
Section 3 – Types of Chemical Reactions
Most chemical reactions can be placed into one of four categories: synthesis,
decomposition, single-displacement, and double-replacement. Each type of reaction has
a pattern that shows how reactants become products.
In a synthesis reaction, two or more substances combine to form one new compound.
You start out with simpler substances (reactants), and you end up with a more complex
substance (product). So something more complex is being synthesized, or built up, from
something less complex. Look carefully at the following examples of synthesis
reactions:
2Na + Cl2  2NaCl
P4 + 3O2  2P2O3
Just the opposite occurs in a decomposition reaction. The word ‘decompose’ means to
break down, and that is exactly what happens in this type of reaction: a single substance
breaks down to form two or more simpler substances; something complex (reactant)
decomposes into its simpler components (products). Here are some examples of
decomposition reactions:
H2CO3  H2O + CO2
2NO2  2O2 + N2
In some reactions, an element replaces another element that is part of a compound. When
this occurs, a single-displacement reaction has occurred. As the following examples
will show, the products of a single-displacement reaction are a new compound and a
different element:
Zn + 2HCl  ZnCl2 + H2
SeCl6 + O2  SeO2 + 3Cl2
In the first equation, on the reactant side zinc is the lone element. However, after the
reaction occurs (the product side), zinc has bonded with chlorine (forming the new
compound ZnCl2), and hydrogen is now the lone element. In the second equation, after
the reaction occurs, oxygen is no longer by itself – it has bonded with selenium and
formed a new compound (SeO2), and chlorine is now the different element.
In a single-displacement reaction, a more reactive element can displace a less reactive
element in a compound, as Figure 4, p. 400 shows in the case of copper and silver. The
only nonmetals that participate in single-displacement reactions are the halogens (group 17).
In a double-displacement reaction, ions from two compounds exchange places. So this
tells us that metals bonded to nonmetals will typically form this type of reaction. One of
the products of a double-displacement reaction is often a gas or a precipitate. Look at the
following examples of a double-displacement reaction:
NaCl + AgF  NaF + AgCl
Na3PO4 + 3KOH  3NaOH + K3PO4
Notice in both equations that the compounds involved in the reactions have switched
bonding partners. For example, in the first equation on the reactant side sodium was
bonded with chlorine, and silver with fluorine. But after these compounds reacted, notice
what we have on the product side: sodium now has bonded with fluorine to form NaF,
and silver has bonded with chlorine to form AgCl.
Section 4 – Energy and Rates of Chemical Reactions
Chemical energy is part of all chemical reactions. Energy is needed to break the
chemical bonds in reactants. As new bonds form in the products, energy is released.
Upon comparing the chemical energy of the reactants and products, it is possible to tell if
energy is released or absorbed in the overall reaction.
An exothermic reaction is a chemical reaction in which energy is released. The prefix
exo means “go out” or “exit.” Energy can be given off in exothermic reactions in several
forms: as light, as electrical energy, or as light and thermal energy (Fig. 1, p. 402).
Energy released in an exothermic reaction is often written as a product:
2Na + Cl2  2NaCl + energy
An endothermic reaction is a chemical reaction in which energy is taken in or absorbed.
The prefix endo means “go in.” Energy taken in during an endothermic reaction is often
written as a reactant:
2H2O + energy  2H2 + O2
The law of conservation of energy states that energy can be neither created nor
destroyed – it can only change forms. Energy can also be transferred from one object to
another, much the same way that a baton is transferred from one runner to another.
What this law means for chemical reactions is this: if you could measure all the energy in
a reaction, you would find that the total amount of energy (of all types) is the same before
and after the reaction.
Just as a bowler must first put in some energy to start the ball rolling down the lane, a
chemical reaction must also get a boost of energy before a reaction can start. This boost
of energy is called the activation energy – the smallest amount of energy that molecules
need to react. Sources of activation energy include friction, as when a match is lit – the
heat produced by the friction of moving the match against the match box strip provides
the activation energy needed to start the reaction. Other possible sources of activation
energy include electricity (as in the spark in a car’s engine that starts it), and light.
A reaction takes place only if the particles of reactants collide. But there must be enough
energy to break the bonds that hold particles together in a molecule, and form new bonds
in new substances. The speed at which new particles form is called the rate of a reaction.
There are four factors that affect the rate of a reaction: temperature, concentration,
surface area, and the presence of an inhibitor or catalyst.
Temperature: At high temperatures, particles of reactants move quickly, and collide often
and with a lot of energy – so, many particles have the activation energy to react. And
many reactants can change into products in a short time.
Concentration: A high concentration of reactants causes a fast rate of a reaction.
Concentration is a measure of the amount of one substance dissolved in another
substance, as shown in Fig. 6, p. 406.
Surface Area: Increasing the surface area of solid reactants increases the rate of a
reaction. For example, grinding a solid into a powder makes a larger surface area –
greater surface area exposes more particles of the reactant to other reactant particles.
This leads to reactant particles colliding with each other more often, and so the reaction
rate is increased.
The Presence of an Inhibitor or Catalyst: An inhibitor is a substance that slows down or
stops a chemical reaction. This can be useful at times, as when preservatives are added to
food to slow the growth of bacteria or fungi. Some antibiotics also serve as inhibitors,
preventing certain kinds of bacteria from multiplying. A catalyst is a substance that
speeds up a chemical reaction without being permanently changed. Because it isn’t
changed, the catalyst is not a reactant. A catalyst lowers the activation energy of a
reaction, which allows the reaction to happen more quickly. Some examples of catalysts
include certain enzymes in your body that speed up chemical reactions, as well as the
catalytic converter in cars.
Information Posted on 3/6/06
Chapter 13 – Chemical Bonding
Section 1 – Electrons and Chemical Bonding
Chemical bonding is the joining of atoms to form new substances. The properties of
these new substances are different from the properties of the original elements. For
example, sodium is a white solid metal, and chlorine is a green, poisonous gas. But when
an atom of sodium reacts with an atom of chlorine, a chemical bond is formed, and a new
substance – sodium chloride, is made. This new substance is white and granular (and
edible!) So sodium chloride has very different properties than either sodium or chlorine.
Electrons in an atom are arranged in energy levels. The first energy level, closest to the
nucleus, can hold up to 2 electrons. The second energy level can hold up to 8 electrons.
The third energy level can hold up to 18 electrons (Fig. 2, p. 365 shows the electron
arrangement for a chlorine atom.
When atoms combine to make chemical bonds, it is almost always the valence electrons
– those electrons in the outermost energy level of an atom – that are responsible for
forming the bond. Determining the number of valence electrons in main group elements
(Groups 1-2, and 13-18) is easy – the element’s group number tells us the number of
valence electrons. For example, all of the elements in group 1 (Alkali Metals) have 1
valence electron. All elements in group 16 (Oxygen Group) have 6 valence electrons
(see Fig. 3, p. 365).
All atoms do not have the same ability to form bonds – some rarely form bonds at all. An
example of atoms that rarely form bonds would be the Noble Gases (Group 18).
Elements in this group have 8 valence electrons (with the exception of helium, which has
2). Atoms like the Noble Gases are in a very stable state, as their outermost energy levels
are full.
Atoms with fewer than 8 valence electrons are more likely to form bonds. Atoms bond
by gaining, losing, or sharing electrons to have a filled outermost energy level – so a
filled outermost level contains 8 valence electrons. As Fig. 5, p. 367 shows, an atom of
sulfur, which has 6 valence electrons, can either gain or share 2 more electrons, thereby
giving it 8 valence electrons. Magnesium has 2 valence electrons. It can have a full
outer level by losing the 2 valence electrons in the third energy level. Now the second
energy level becomes the outermost energy level and has 8 electrons. Light elements like
helium, hydrogen, lithium and beryllium will have full outermost energy levels with only
2 valence electrons when reacting with other atoms. For example, lithium, upon
reacting with another atom, will lose its one valence electron in the second energy level,
and the first energy level now becomes its outermost energy level, holding 2 valence
electrons. As we shall see in section 2, when reacting with other atoms, metals (like
magnesium) will give up electrons, and non-metals (like sulfur) will receive electrons.
Section 2 – Ionic Bonds
An ionic bond forms when electrons are transferred from one atom to another. During
ionic bonding, one or more valence electrons are transferred from one atom to another.
Like all chemical bonds, ionic bonds form so that the outermost energy levels of the
atoms in the bonds are filled.
An atom is neutral because the number of protons equals the number of electrons – the
positive and negative charges cancel each other out, giving the atom an overall charge of
0. However, when ionic bonds form, atoms are transferring electrons – either giving
them away, or receiving them. The number of protons, of course, stays the same, but
now the number of protons and electrons is different – the charges don’t cancel out, and
the atoms become ions – charged particles that form when atoms gain or lose electrons.
Since metals have few valence electrons, it’s easier for them to lose those valence
electrons when reacting to form bonds. Only a small amount of energy is needed to take
electrons from metal atoms. Non-metals, on the other hand, have more valence electrons,
and so it would be easier for them to gain what little electrons they need to have a full
outermost energy level. Look at Figure 2, p. 369. Sodium, a metal, has 11 protons and
11 electrons. But during chemical changes, sodium will lose its 1 valence electron – it
now has 11 protons and 10 electrons – 11 minus 10 is positive 1. Now we no longer have
a neutral sodium atom, but rather a positively charged sodium ion (Na+). For aluminum,
since this metal has 3 valence electrons, when it loses them upon reacting, the aluminum
ion is written as Al3+, since the original aluminum atom lost 3 electrons (13 protons
minus 10 electrons equals positive 3).
As we’ve seen, metals like sodium and aluminum for positive ions when reacting, since
they give away electrons. Non-metals, however, form negative ions, since they receive
electrons upon reacting with another atom. Look at Figure 3, p. 370. Oxygen, with 8
protons and 8 electrons, becomes an oxide ion (O2-) when reacting, since it receives two
electrons from the metal that its forming a bond with (8 protons minus 10 electrons
equals -2). Chlorine, with 7 valence electrons, gains the one electron it needs to have a
full outermost energy level when reacting with a metal such as sodium. The chlorine
atom, originally with 17 protons and 17 electrons, now becomes a chloride ion (Cl-) with
17 protons and 18 electrons.
Energy is given off by most nonmetal atoms when they gain electrons – the more easily
an atom gains an electron, the more energy the atom releases. So atoms of Group 17 (the
halogens) give off the most energy when they gain an electron. An ionic bond will form
between a metal and a nonmetal if the nonmetal releases more energy than it takes to take
electrons from the metal. And since it takes only a small amount of energy to pull the
few valence electrons from metal atoms, forming ionic bonds isn’t hard.
When ionic bonds form, the number of electrons lost by the metal atoms equals the
number gained by the nonmetal atoms. The ions that bond are charged, but the
compound formed is neutral because the charges of the ions cancel each other out. For
example, when sodium and chlorine react, the compound sodium chloride is formed. The
ionic configuration is Na+Cl-. But since the charges are equal in number (1) but opposite,
they cancel, so the compound NaCl is neutral. When ions bond, they form a repeating
three-dimensional pattern called a crystal lattice, like the sodium chloride (table salt)
shown in Figure 4, p. 371.
Section 3 – Covalent and Metallic Bonds
A covalent bond forms when atoms share one or more pairs of electrons. When two
atoms of nonmetals (such as hydrogen) bond, they do so by sharing electrons with each
other – see Fig. 1, p. 372). So unlike ionic bonds, covalent bonds DO NOT form by the
losing and gaining of electrons – rather the atoms that make up a covalent bond SHARE
the valence electrons that are used to form the bond. For example, look at the diagram
below, which shows how two atoms of hydrogen bond to form molecular hydrogen (H2).
Notice how each electron from the hydrogen atoms are shared equally by the two
molecules – this is referred to as a nonpolar covalent bond. In the other type of covalent
bond – polar covalent bonds - there is an unequal sharing of electrons among the atoms
that form the bond, such as in the water molecule (H2O). Because oxygen is a more
massive atom than hydrogen, it tends to pull electrons towards it, leaving the oxygen
atom with a slight negative charge. This results in the two hydrogen atoms having a
slight positive charge, as shown below:
Notice in both cases that all atoms in the bonds have full valence levels – hydrogen with
two, and oxygen with eight.
There are a couple of ways to represent atoms and molecules that can form covalent
bonds. One way is the ball and stick model, in which atoms are joined together with one
stick (representing a single bond), two sticks (representing a double bond), or three sticks
(representing a triple bond). The number of valence electrons needed by the atoms to
have a complete valence level will determine what kinds of bonds are formed. The other
method is known as the electron dot diagram, which represents atoms and molecules
with dots drawn around them, representing the number of valence electrons. To write an
electron dot diagram for a single atom, first write the element’s symbol. Then, starting at
the top of the element, write single dots (each dot representing a valence electron). If the
element has more than 4 valence electrons, such as oxygen with 6, then any remaining
dots are paired up. For molecules, paired dots are drawn between the atoms making up
either polar or nonpolar covalent bonds (see Fig. 2 & 3, p. 373).
Substances containing covalent bonds consist of individual particles called molecules –
two or more atoms joined in a definite ratio. Molecules are composed of at least two
covalently bonded atoms. Diatomic molecules are made up of two atoms of the same
element (examples would be H2, O2, N2 and the halogens I2, F2, Br2 and Cl2). Look at
the fluorine molecule in Fig. 5, p. 374 – note that the shared electrons are counted as
valence electrons for each atom. So, both atoms of the molecule have filled outermost
(valence) energy levels. There are more complex molecules as well, such as the proteins
in your body. Carbon atoms are the basis of many of these more complex molecules.
Each carbon atom needs to make 4 covalent bonds to have 8 valence electrons – these
bonds can be with other atoms of other elements or with other carbon atoms, as shown in
the model of the sugar sucrose (Fig. 6, p. 375).
A metallic bond is formed by the attraction between positively charged metal ions and
the electrons in the metal. Positively charged metal ions form when metal atoms lose
electrons. Bonding in metals is the result of metal atoms being so close to one another
that their outermost energy levels overlap. This overlapping allows valence electrons to
move throughout the metal (Fig. 8, 376). So a metal can be thought of as being made up
of positive metal ions that have enough valence electrons ‘swimming’ around to keep the
ions together. The electrons also cancel the positive charge of the ions.
Metallic bonding is what gives metals their properties of electrical conductivity (ability to
conduct electricity, such as copper in wiring), malleability (ability to be hammered into
sheets, as in aluminum), and ductility (ability to be drawn into wires (such as copper).
The bonds formed in metals prevent them breaking. When a piece of metal is bent, some
of the metal ions are forced closer together. One might expect the metal to break because
all of the metal ions are positively charged. But since these ions are surrounded by a sea
of electrons, no breaking occurs. It’s theses moving electrons around and between the
metal ions that maintain the metallic bonds, no matter how the shape of the metal changes
(Fig. 9, p. 377).
Information Posted on 2/13/06
Chapter 12 – The Periodic Table
Section 1 – Arranging the Elements
In the early 1860’s, scientists knew of the properties of more than 60 elements – but no
one had organized the elements according to those properties. When this was eventually
done, scientists would be greatly aided in their understanding of how elements interact
with each other. It was Russian chemist Dmitri Mendeleev who is credited with first
discovering a pattern to the elements in 1869. He arranged the elements in order of
increasing atomic mass. When he did so, he noticed a pattern emerging. When the
elements were arranged in order of increasing atomic mass, those with similar properties
occurred in a repeating pattern – in other words, the pattern was periodic. Periodic
means “happening at regular intervals.” Mendeleev found that the elements’ properties
followed a pattern that repeated every seven elements, and his table became known as the
periodic table of the elements. Figure 2, p. 337 shows his first try at arranging the
elements.
In 1914, Henry Moseley, a British scientist, determined the number of protons-the atomic
number—in an atom. All elements fit the pattern in Mendeleev’s periodic table when
they were arranged by atomic number. Looking at the periodic table in your textbook on
p. 338-339, all of the more than 30 elements discovered since 1914 follow the periodic
law, which states that the repeating chemical and physical properties of elements change
periodically with the elements’ atomic numbers.
Elements are classified as metals, nonmetals, and metalloids, according to their
properties. The zigzag line on the periodic table helps one recognize which elements are
metals, which are nonmetals, and which are metalloids. Looking at your periodic table’s
key, it’s clear that most of the elements are metals – these are found on the left side of the
periodic table. Atoms of most metals have few electrons in their outer energy level, and
are solid at room temperature (mercury is a notable exception, being a liquid at room
temperature). As Figure 3, p. 340 illustrates, some other properties of metals are that
they are shiny, malleable (can be flattened with a hammer), ductile (can be drawn through
a wire), and are good conductors of thermal energy. Nonmetals are found on the right of
the zigzag line on the periodic table. Atoms of most nonmetals have an almost complete
set of electrons in their outer energy level. Group 18 elements, the noble gases, have a
complete set of electrons. More than half of the nonmetals are gases at room
temperature, and they have properties that are the opposite of metals, as shown in Figure
4, p. 341. Metalloids (semiconductors) border the zigzag line on the periodic table.
Atoms of metalloids have about half of a complete set of electrons in their outer energy
level, and they have properties of metals and some properties of nonmetals, as shown in
Figure 5, p. 341.
Each square on the periodic table includes the element’s name, chemical symbol, atomic
number, and atomic mass. Each element is identified by a chemical symbol. Names of
elements come from a variety of sources, such as scientists (i.e., mendelevium) and
places (i.e., californium). Some names have their roots in other languages. For example,
the element sodium’s symbol is Na. This is from the Latin word ‘Natrium.’ The first
letter of a chemical symbol is always capitalized, and any other letter is always
lowercase.
Each horizontal row of elements (left to right) on the periodic table is called a period.
There are seven periods on the periodic table. Look at the period 4 elements in Figure 6
on p. 342. The physical and chemical properties of elements in a row follow a repeating,
or periodic, pattern as you move across the period. Properties such as conductivity and
reactivity change gradually from left to right in each period. Additionally, all of the
elements in a row have the same number of energy levels. Each vertical column of
elements (top to bottom) on the periodic table is called a group. There are 18 groups on
the periodic table. Elements in the same group have the same physical and chemical
properties, and for this reason a group is also called a family. Elements in a group also
have the same number of valence electrons – the number of electrons in their outermost
energy level. So all elements in group 2 have 2 electrons in their outermost energy level.
For groups 13-18, you subtract 10 from the group number to get the number of valence
electrons. So elements in group 13 would have 3 valence electrons, group 17 elements
would have 7 valence electrons, etc.
Section 2 – Grouping the Elements
In this section we will look at the particular groups of elements that make up the periodic
table. Starting at the far left, we have Group 1, the Alkali Metals. The elements in
group 1 (Fig. 1, p. 344) are the most reactive metals because their atoms can easily give
away the one electron in their outer energy level. For example, when sodium and
chlorine react together to yield the compound sodium chloride, sodium gives up its one
electron in its outer energy level to chlorine, which, being a nonmetal, receives electrons.
Group 2 elements (Fig. 2, p. 345) are the Alkaline-Earth Metals. Atoms of alkalineearth metals have 2 electrons in their outer energy level. Because it is more difficult for
atoms to give two electrons than to give one when joining with other atoms, the alkalineearth metals are less reactive than the alkali metals. Group 2 elements and compounds
formed from them have many uses; for example, in making low-density materials used in
airplanes.
Groups 3-12 do not have individual names, but are collectively referred to as the
transition metals. The atoms of transition metals do not give away their electrons as
easily as atoms of the Group 1 and Group 2 metals do. Therefore, transition metals are
less reactive than both alkali metals and alkaline-earth metals are. Properties of
transition metals vary widely, as shown in Figure 3, p. 346. But being metals, they share
the properties of metals (i.e., shiny, good conductors, etc.).
The Lanthanides and Actinides (p. 346), which are actually transition metals from periods
6 and 7, appear in two rows at the bottom of the periodic table to keep the table from
being too wide. Lanthanides are elements in the first (top) row. They are shiny, reactive
metals, some of which are used to make steel. Second (bottom) row elements are called
actinides. All atoms of actinides are radioactive, or unstable. The atoms of a radioactive
element can change into another element. Many of the actinides are not found in nature,
but rather are made in laboratories. Americium (element 95), for example, is used in
some smoke detectors.
Group 13 is the Boron Group (p. 347). Aluminum, besides being the most common
element in this group, is also the most abundant metal in Earth’s crust. Elements in the
boron group are reactive. Note in your text that boron is a metalloid, and the rest of
group 13 elements (Al, Ga, In, Tl) are metals. Group 14 is the Carbon Group (p. 347),
and it contains a nonmetal (C), two metalloids (Si, Ge), and three metals (Sn, Pb, Uuq).
Carbon forms many important compounds found in living things. Group 15 is the
Nitrogen Group (p. 348). There are two nonmetals (N, P), two metalloids (As, Sb), and
one metal (Bi) in this group. Nitrogen, a gas at room temperature, makes up about 78%
of the Earth’s atmosphere. Group 16 is the Oxygen Group (p. 348). It comprises three
nonmetals (O, S, Se), one metalloid (Te), and a metal (Po). Oxygen makes up about 20%
of the atmosphere. Group 17 is the Halogens (p. 349). These are very reactive
nonmetals because their atoms need to gain only one electron to have a complete outer
level (when an atom has 8 electrons in its outermost level, it is complete). All elements
in this group are nonmetals (F, Cl, Br, I, At). Group 18 are the Noble Gases (p. 350).
This group contains only nonmetals (He, Ne, Ar, Kr, Xe, Rn). Noble gases are nonreactive metals – they do no need to lose or gain electrons, as they have a full set of
electrons in their outer level. The properties of hydrogen (p. 350) do not match the
properties of any single group, so it is set apart from the other elements in the periodic
table. Hydrogen is above Group 1 because atoms of the alkali metals also have one
electron in their outer level. Hydrogen is reactive.
Information Posted on 2/1/06
Chapter 11 –Introduction to Atoms
Section 1 – Development of an Atomic Theory
Since ancient times, humankind has wondered how the natural world was put together.
Some Greek philosophers, like Aristotle, thought that you could keep cutting up matter
forever, never ending up with a particle that couldn’t be cut further. Another
philosopher, however, named Democritus, thought that you would eventually end up with
a particle that could not be cut. He called this particle atomos, meaning “not able to be
divided.” Democritus turned out to be right: matter is made of particles called atoms,
which are the smallest particles into which an element can be divided and still be the
same substance.
Since the time of Democritus, the development of atomic theory experienced a gradual
progression of ideas that would eventually lead to the modern atomic theory. One
advantage that later scientists had over philosophers like Democritus is that they could
test their ideas. In the late 1700’s, scientists learned that elements combine in certain
proportions based on mass to make compounds. In 1803, John Dalton, a British chemist
and teacher, wanted to know why, and so began experimenting with different substances.
Following results that he obtained, Dalton published his atomic theory, which stated that:
(1) all substances are made of atoms. Atoms are small particles that cannot be created,
divided, or destroyed. (2) Atoms of the same element are exactly alike, and atoms of
different elements are different. (3) Atoms join with other atoms to make new
substances.
While Dalton’s ideas were largely correct, new data would be obtained toward the end of
the 1800’s that didn’t fit with some of Dalton’s ideas. The stage was set for atomic
theory to evolve further. In 1897, British scientist J.J. Thomson pointed out a mistake in
Dalton’s theory, showing by experiment that there were particles inside the atom. This
meant that atoms can be divided into even smaller parts. From Thomson’s experiment
(see Fig. 3, p. 314), he discovered electrons – particles with a negative charge. Thomson
proposed a new model of the atom, sometimes called the plum-pudding model, because
Thomson thought that electrons were mixed throughout an atom like plums in a pudding.
Here is a diagram of Thomson’s model:
In 1909, Ernest Rutherford tested Thomson’s theory. His famous ‘gold-foil experiment’
is shown on p. 315, Figure 5. Starting out with Thomson’s idea that atoms are soft
‘blobs’ of matter, he expected the particles to pass right through the gold. Most of the
particles did just that. But to his surprise, some of the particles were deflected, or even
bounced straight back. He realized that in order to explain this, atoms must be
considered mostly empty space, with a tiny part made of highly dense matter. Figure 7,
p. 316 gives you an idea for the dimensions of an atom. In 1911, Rutherford revised the
atomic theory, proposing that in the center of the atom was a tiny, extremely dense
positively charged part called the nucleus. Here is a diagram of Rutherford’s model:
In 1913, Niels Bohr, a Danish scientist who worked with Rutherford, studied the way that
atoms react to light. His results led him to propose that electrons move around the
nucleus in certain paths, or energy levels. In Bohr’s model there are no paths between
levels. But electrons can jump from a path in one level to a path in another level. His
model was a valuable tool in predicting some atomic behavior, but there was still room
for some improvement. Here is a diagram of Bohr’s model:
Many 20th century scientists added to our current understanding of the atom. Physicists
Erwin Schrodinger and Werner Heisenberg did very important work, further explaining
the nature of electrons in an atom. Their work showed that electrons do not travel in
definite paths as Bohr suggested. According to the current theory, often called the
Electron Cloud Model (see diagram below), there are regions inside the atom where
electrons are likely to be found. These regions are called electron clouds.
Section 2 – The Atom
The dimensions of atoms are difficult to comprehend. To give you some idea of just how
small they are, a piece of aluminum foil is about 50,000 aluminum atoms thick! As tiny
as an atom is, it is made of even smaller particles. Those particles are protons, neutrons,
and electrons. Here is a diagram of the atom and its parts:
The nucleus is the small dense, positively charged center of the atom. It contains most of
the atom’s mass. Inside the nucleus are found two type of particles; protons and
neutrons. Protons are positively charged particles. Neutrons are particles that do not
have a charge; they are neutral; hence the name ‘neutron.’ Outside of the nucleus are
found electrons – particles that have a negative charge. Electrons are found in electron
clouds outside the nucleus. It is the size of the electron clouds that determine the size of
the atom. In the diagram above, the nucleus is enlarged to show you the protons and
neutrons clearly. However, in reality the diameter of the nucleus is 1/100,000 the
diameter of the atom. Remember that an atom is mostly empty space, with the nucleus
occupying only a tiny part of it.
In terms of mass, scientists use a unit called the atomic mass unit to express masses of
particles. Each proton has a mass of about 1.7 x 10-24 grams, or 1 amu. Neutrons are a
little more massive than protons, although the difference is so small that the mass of a
neutron is also give as 1 amu. Electrons, compared with protons and neutrons, are very
small in mass. It takes more than 1,800 electrons to equal the mass of one proton.
The charges of protons and electrons are opposite but equal, so their charges cancel out.
Because an atom has no overall charge, it is neutral. Atoms can be stripped of electrons,
or they can gain electrons. In this case the balance between protons and electrons is lost,
and an ion is formed. An atom that loses one or more electrons becomes a positively
charged ion, and an atom that gains one or more electrons becomes a negatively charged
ion.
Atoms of different elements differ because they have different numbers of protons in the
nucleus – the number of protons in nucleus of an atom is called the atomic number of
that atom. All atoms of an element have the same atomic number. For example, every
carbon atom has only 6 protons in its nucleus. However, it is possible for atoms of the
same element to differ in the number of neutrons that they have. Isotopes are atoms that
have the same number of protons but have different numbers of neutrons. So atoms that
are isotopes of each other are always the same element, because isotopes always have the
same number of protons. The different numbers of neutrons, however, gives them
different masses. The isotopes of hydrogen are written as hydrogen-1, hydrogen-2, and
hydrogen-3. See Figure 4, p. 321 for a diagram of two isotopes of hydrogen.
You can identify each isotope of an element by its mass number. The mass number is
the sum of the protons and neutrons in an atom. For example, as Figure 5, p. 322 shows,
one isotope of boron has 5 protons and 6 neutrons, and 5 electrons. So the mass number
of this boron isotope is 5 + 6 = 11. Electrons are not included in the mass number
because their mass is so small, and therefore have very little effect on the atom’s total
mass. The carbon isotope with a mass number of 12 is called carbon-12. Knowing that
the atomic number for carbon is 6, you can find the number of neutrons in carbon-12 by
subtracting the atomic number from the mass number. For carbon-12, the number of
neutrons is 12 – 6, or 6.
The atomic mass of an element is the weighted average of the masses of all the naturally
occurring isotopes of that element. A weighted average accounts for the percentages of
each isotope that are present. For example, Chlorine-35 makes up 76% of all the chlorine
in nature, and chlorine-37 makes up the other 24%. To find the atomic mass of chlorine,
just multiply the mass number of each isotope by its percentage abundance in decimal
form, then add those amounts together:
(35 x 0.76) = 26.60
(37 x 0.24) = +8.88
_______
35.48 amu
There are forces acting within the atom (Figure 7, p. 324). These forces are the
gravitational force, electromagnetic force, strong force, and weak force. Gravity is the
weakest force, and so the gravitational force plays the virtually no role on scales the size
of atoms, because the masses of particles in atoms are so small. The electromagnetic
force holds the electrons around the nucleus. The strong force is responsible for holding
the nucleus together – otherwise the protons in the nucleus would fly apart, since they are
positively charged, and like charges repel each other. The weak force is an important
force in radioactive atoms. In certain unstable atoms, a neutron can change into a proton
and an electron – the weak force makes this possible.
Notes for Quarter 2
Information Posted on 1/18/06
(Temperature and Heat Notes for Chapter 10)
Section 1 - Temperature
Temperature is a measure of the average kinetic energy of the particles in an object. All
matter is made of atoms or molecules in motion – the faster the particles are moving, the
more kinetic energy they have, and the more kinetic energy the particles of the object
have, the higher the temperature of the object is (see Fig. 1 p. 274).
When you measure the temperature of an object, you are measuring the average kinetic
energy of all the particles in the object – the particles are moving around randomly at
different speeds, and so have different amounts of kinetic energy. So the average kinetic
energy of the particles is measured, and that is what is registered as a certain temperature.
A thermometer is used to measure temperature. Most thermometers are thin glass tubes
filled with a liquid, usually mercury or alcohol, since they remain liquid over a wide
range of temperatures (although mercury is not used as widely today because of safety
concerns). Thermometers can measure temperature because of a property called
thermal expansion – the increase in volume of a substance because of an increase in
temperature. As a substance’s temperature increases, its particles move faster and spread
out – so there is more space between them, and the substance expands. Mercury and
alcohol expand by constant amounts for a given change in temperature.
There are three principle scales used to measure temperature; Fahrenheit, Celsius, and
Kelvin (see Fig. 3, p. 276 – you should be familiar with the boiling and freezing points
for the three scales). Absolute zero is the lowest temperature on the Kelvin scale (0 K).
Absolute zero (about -459oF), is the temperature at which all molecular motion stops.
Section 2 – Heat
Heat is the energy transferred between objects that are at different temperatures. When
two objects at different temperatures come into contact, energy is always transferred from
the object that has the higher temperature to the object that has the lower temperature.
The type of energy that is transferred is called thermal energy - the total kinetic energy
of the particles that make up a substance. Measured in joules (J), thermal energy
depends partly on temperature (something at a high temperature has more thermal
energy), and partly on how much of a substance there is (the more particles there are in a
substance at a given temperature, the greater the thermal energy of the substance is).
There are three ways of energy is transferred. Thermal conduction is the transfer of
thermal energy from one substance to another through direct contact (a metal spoon in a
hot bowl of soup displays conduction). Substances that conduct thermal energy well are
called thermal conductors, and those that do not are called thermal insulators (see
Table 1, p. 283). Convection involves the transfer of thermal energy by the circulation
or movement of a liquid or gas (the rising and sinking of water during boiling is an
example of convection). The third way that energy is transferred is radiation – the
transfer of energy by electromagnetic waves, such as visible light, infrared, or other types
of radiation in the electromagnetic spectrum). Energy from the sun is a form of radiation.
Information Posted on 1/17/06
Notes for Chapter 9 – Energy and Energy Resources
Section 1 – What Is Energy?
Energy is the ability to do work. Work is done when a force causes an object to move in
the direction of the force. As Figure 1, p. 240 shows, the tennis player does work on her
racket by exerting a force on it. The racket, in turn, does work on the ball, and the ball
does work on the net. When one object does work on another, energy is transferred from
the first object to the second object – this allows the second object to do work. So, work
is a transfer of energy.
In our tennis example, energy is transferred from the racket to the ball. As the ball flies
over the net, the ball has kinetic energy – energy of motion. An object’s kinetic energy
can be found by the following equation:
Kinetic energy =
Mv2
---------2
The m stands for mass in kilograms. The v represents an object’s speed – the faster
something is moving, the more kinetic energy it has. Also, the greater the mass of a
moving object, the greater its kinetic energy is. But notice that the speed (v) in the
equation is squared. So speed has a greater effect on kinetic energy than mass does. Car
crashes are more dangerous at higher speeds than at lower speeds. A moving car has 4
times the kinetic energy of the same car going half the speed! Look at the example in the
Math Focus section on p. 241 to see this equation used in a real problem.
Not all energy has to do with motion. Potential energy is the energy an object has
because of its position. As Figure 3 p. 242 shows, a stretched bow has potential energy –
the bow has energy because work has been done to change its shape. The energy of that
work is turned into potential energy. Gravitational potential energy is a special case that
occurs when you lift an object. As the object is lifted, work is being done on it – you use
a force that is against the force of gravity. When you do this, you transfer energy to the
object and give the object gravitational potential energy. You can find gravitational
potential energy by using the following equation:
Gravitational potential energy = weight x height
Mechanical energy is the total energy of motion and position of an object. Both potential
energy and kinetic energy are kinds of mechanical energy. Mechanical energy can be all
potential energy, all kinetic energy, or some of each (see Fig. 4, p. 243). You can find
mechanical energy by using the equation:
Mechanical energy = potential energy + kinetic energy
There are other forms of energy as well (see p. 244-245 for detailed descriptions).
Thermal energy is all of the kinetic energy due to random motions of the particles that
make up an object. Chemical energy is the energy of a compound that changes as its
atoms are rearranged. Electrical energy is the energy of moving electrons. Sound energy
is caused by an object’s vibrations. Light energy is produced by the vibrations of
electrically charged particles. Nuclear energy comes from changes in the nucleus of an
atom.
Section 2 – Energy Conversions
Energy conversion is a change from one form of energy to another. Any form of energy
can change into any other form of energy. Often, one form of energy changes into more
than one form. Figure 1, p. 248 shows how the skateboarder changes from potential to
kinetic energy, depending on his position in the skate arena. When he’s at the very top,
his potential energy is at maximum. As he speeds through the bottom, his kinetic energy
is at maximum. A rubber band provides another example of energy conversion.
Stretching a rubber band takes a little effort. The energy that you put into it becomes
elastic potential energy. When its let go, the rubber band snaps back to its original shape
– it releases its stored up potential energy as it does so.
Many other examples of energy conversations can be put forth. Chemical energy of food
is converted into kinetic energy when you are active, plus thermal energy to help
maintain body temperature. In photosynthesis in plants, light energy is converted into
chemical energy (Fig. 4, p. 250). The chemical energy from a tree can be changed into
thermal energy if you burn it.
Energy conversions are needed for everything that we do. For example, a hairdryer (Fig.
5, p. 251) converts electrical energy into thermal energy to dry your hair. Sound energy
is also produced, which you can hear. Look through Table 1, p. 251 for other examples
of electrical energy conversions.
Machines of course use energy – a machine makes work easier by changing the size or
direction (or both) of the force needed to do the work. As Figure 6, p. 252 shows, when
using a nutcracker, some of the energy you transfer to the nutcracker is converted to
sounds energy as the nutcracker transfers energy to the nut. Figure 7 shows energy
conversions that occur in the act of riding a bicycle. Machines help us use energy by
converting it into the form that you need.
Section 3 – Conservation of Energy
Look at Figure 1, p. 254. As the cars go up and down the hills on the track, potential
energy get converted into kinetic energy, and back again. But the cars never return to the
same height that they started at. But energy is not lost along the way, just converted into
other forms of energy. The original potential energy of the roller coaster is lost in part to
friction – a force that opposes motion between two surfaces that are touching. For the
roller coaster to move, energy must be used to overcome friction. There is friction
between the cars’ wheels and the track, as well as between the cars and the air around
them. So because of this, not all the potential energy gets converted into kinetic energy
as the cars go down the hill. The roller coaster is an example of a closed system – a
group of objects that transfer energy to each other.
Energy is conserved in all cases – no exception to this has ever been found, and so this
rule is described as a law. According to the Law of Conservation of Energy, energy
cannot be created or destroyed – it can only be changed from one form to another. The
total amount of energy in a system is always the same. See Figure 2 p. 255 for a diagram
detailing energy conservation in a light bulb.
Any time one form of energy is converted into another form, some of the original energy
always gets converted into thermal energy – the thermal energy due to friction that results
from energy conversions is not useful energy – not used to do work. In a car, not all of
the gasoline’s chemical energy makes the car move. Some wasted thermal energy will
always result from the energy conversions, leaving through the radiator and tail pipe.
Because of this, it is impossible to build a perpetual motion machine (Fig. 3, p. 256).
Such a machine would put out exactly as much energy as it takes in.
While we can’t build machines that would run forever without any additional energy, we
can strive to make machines (such as cars) more energy efficient. In terms of energy
conversions, energy efficiency is a comparison of the amount of energy before a
conversion with the amount of useful energy after a conversion. So a car with high
energy efficiency can go farther than other cars with the same amount of gas.
Section 4 – Energy Resources
An energy resource is a natural resource that can be converted into other forms of energy
in order to do more useful work. Some energy resources, called nonrenewable
resources, cannot be replaced or are replaced much more slowly than they are used.
Fossil fuels are the most important nonrenewable resources. Fossil fuels are energy
resources that formed from the buried remains of plants and animals that lived millions of
years ago. These plants stored energy from the sun by photosynthesis. Animals used and
stored that energy by eating the plants. So, fossil fuels are concentrated forms of the
sun’s energy (see Fig. 1, p. 258 to see how fossil fuels are formed). Today, millions of
years later, energy from the sun is released when these fossil fuels are burned.
As Figure 2, p. 259 shows, fossil fuels can be used in different ways in society. Energy
can be generated by burning the three main types of fossil fuels – coal, petroleum, and
natural gas. Electrical energy is one of the main types of power derived, or obtained,
from fossil fuels. For example, electric generators convert the chemical energy in fossil
fuels into electrical energy by process shown in Figure 3, p. 260. Nuclear energy can
also be used to generate electrical energy. Like fossil-fuel power plants, a nuclear power
plant generates thermal energy that boils water to make steam. The steam then turns a
turbine, which runs a generator. The spinning generator converts kinetic energy into
electrical energy. In nuclear power plants, nuclear energy is generated from radioactive
elements such as uranium. In a process called nuclear fission, the nucleus of the uranium
atom is split into two smaller nuclei, which releases nuclear energy.
Renewable resources are naturally replaced more quickly than they are used. Examples
of renewable resources include solar energy, energy from water, wind energy, biomass,
and geothermal energy (see p. 261-262). Sunlight can be changed into electrical energy
through solar cells. The potential energy of water in a reservoir can be changed into
kinetic energy as the water moves through a dam. This falling water turns turbines,
which are connected to a generator that changes kinetic energy into electrical energy. The
kinetic energy of wind can turn the blades of a windmill. A wind turbine changes the
kinetic energy of the air into electrical energy by turning a generator. Geothermal energy
is thermal energy caused by the heating of the Earth’s crust. Some geothermal power
plants pump water underground next to hot rock. The water turns to steam, which can
then turn the turbine of a generator. Finally, biomass - organic matter such as plants,
wood, and waste can be burned to release energy.
As Table 1, p. 262 shows, there are both advantages and disadvantages to using these
various sources of renewable and nonrenewable energy resources to generate other forms
of energy.
Information Posted on 1/6/06
Notes for Chapter 8 – Work and Machines
Section 1 – Work and Power
Work is done when a force causes an object to move in the direction of the force.
Applying a force doesn’t always result in work being done – pushing a stalled car that
doesn’t budge – although a tiring task – doesn’t result in any work being done because
the car hasn’t moved. If the car moves, however, then it could be said that work had been
done on the car – because the car would move in the same direction as the force being
applied by you on it (see Fig. 2, p. 211 for both examples and non-examples of work).
Work depends on distance as well as force. Look at the climbers in Figure 4 on p. 212.
The climbers on the left are walking up a slope, and those on the right are going straight
up the cliff. In which case is more work being done? Well, the same amount of work is
being done in both cases. Why? Because of the relationship between force and distance.
The climbers who walk up the slope don’t need to use as much force as the climbers who
go straight up the cliff. But the climbers walking up the slope have to go farther than the
climbers going straight up the cliff. In the first case, less force being applied over a
greater distance, and in the second case you have more force applied over a shorter
distance. But in both cases the amount of work is the same.
The amount of work (W) done in moving an object can be calculated by multiplying the
force (F) applied to the object by the distance (d) through which the force is applied:
W=Fxd
Force is expressed in newtons, the meter is the basic metric unit for length or distance.
So the unit used to express work is the newton-meter (N x m), or simply the joule.
Power is the rate at which energy is transferred. You calculate power (P) by dividing the
amount of work done (W) by the time (t) that it takes to do that work:
W
P = ---t
The unit used to express power is joules per second (J/s), or the watt. One watt is equal
to 1 J/s. How can power be increased? If you sand a shelf by hand (Fig. 6, p. 214), then
the power output is lower. An electric sander can do the same amount of work faster –
therefore, the electric sander has more power.
Section 2 – What is a Machine?
A machine is a device that makes work easier by changing the size or direction of a
force. While we often think of complex things such as cars and computers as machines,
we use many much more simpler devices in our everyday lives that qualify as machines
as well. We can understand how a machine functions in terms of two important concepts;
work input and work output. The work that you do on a machine is called work input.
The force that you apply to the machine through a distance is called the input force. The
work done by the machine on an object is called work output. The force that the
machine applies through a distance is called the output force.
It is important to understand that work output can never be greater than work input.
Looking at Figure 2, p. 217, if you multiplied the forces by the distances through which
the forces are applied (recall that W = F x d), you’d find that the screwdriver does not do
more work on the lid (output force) than you do on the screwdriver (input force). So
machines allow force to be applied over a greater distance, which means that less force
will be needed for the same amount of work. Look at Figure 3, p. 218. Lifting the box
straight up from the ground will require a greater input force applied over a shorter
distance. Pushing it up the ramp, however, will involve less input force applied over a
greater distance. But notice something significant – the same amount of work – 450J – is
done.
Mechanical advantage is the number of times the machine multiplies force. Put another
way, the mechanical advantage of a machine compares the input force with the output
force:
Mechanical advantage (MA) =
Output force
--------------------Input force
Using this equation, if you had to push a 500 N weight up a ramp and only needed to
push 50 N of force the entire time, than the mechanical advantage of the ramp would be
10 (500 N / 50 N = 10). A machine that has a mechanical advantage greater than 1 will
help move or lift heavy objects because the output force (the work that the machine does
on the object) is greater than the input force (the work that you do on the machine).
Mechanical efficiency is a comparison of a machine’s work output with the work input.
A machine’s mechanical efficiency is calculated using the equation:
Mechanical efficiency =
Work output
---------------- x 100
Work input
The 100 in the equation means that mechanical efficiency is expressed as a percentage.
So mechanical efficiency tells you what percentage of the work input gets converted into
work output. While an ideal machine would have 100% mechanical efficiency, this is
unfortunately impossible. Every machine has moving parts, and these moving parts
always use some of the work input to overcome friction. But new technologies
continuously increase machine efficiency so that more energy is available to do useful
work (see Fig. 6, p. 221).
Section 3 – Types of Machines
In this section, we looked at six simple machines, as well as machines composed of two
or more simple machines called complex machines. A lever is a simple machine that has
a bar that pivots at a fixed point called a fulcrum. Levers are used to apply a force to a
load. There are three classes of levers, which are based on the locations of the fulcrum,
the load, and the input force (see the figures on p 222-223 for examples of the three
classes of levers). A pulley is a simple machine that has a grooved wheel that holds a
rope or cable. A load is attached to one end of the rope, and input force is applied to the
other end. There are three types of pulleys - fixed, movable, and block and tackle. Study
Figure 4 on p. 224 to see how these three types of pulleys operate, the relationship
between input force and output force, and the mechanical advantage is each.
The wheel and axle is a simple machine that has two circular objects of different sizes
(see Fig. 5 p. 225). Doorknobs, steering wheels, and wrenches all use a wheel and axle.
The mechanical advantage of a wheel and axle is the radius of the wheel divided by the
radius of the axle (see Fig. 6 p. 225). An inclined plane is a simple machine that is a
straight, slanted surface. A ramp would be a good example of an inclined plane. The
mechanical advantage of an inclined plane is found by dividing the length of the inclined
plane by the height to which the load is lifted (see Fig. 7 p. 226 – the inclined plane in the
picture has a mechanical advantage of 5, making it relatively easy to push the piano into
the truck).
A wedge is a pair of inclined planes that move. A knife, plows, axes and chisels are all
examples of wedges. Since wedges are useful for cutting, the longer and thinner the
wedge is, the greater its mechanical advantage – this is why axes and knives cut better
when you sharpen them – you are making the wedge thinner. The mechanical advantage
of a wedge is found by dividing the length of the wedge by its greatest thickness (see Fig.
8 p. 227). A screw is an inclined plane that is wrapped in a spiral around a cylinder, as
shown in Fig. 9, p. 227). When a screw is turned, a small force is applied over the long
distance along the inclined plane of the screw. Meanwhile, the screw applies a large
force through the short distance it is pushed. The longer the spiral on a screw is and the
closer together the threads are, the greater the screw’s mechanical advantage is. A jar lid
is a screw that has a large mechanical advantage.
Compound machines consist of two or more simple machines. A block and tackle is
actually a complex machine, since it consists of two or more pulleys. A can opener (Fig.
10, p. 228) is also a compound machine consisting of three simple machines: the handle
is a second-class lever, the knob is a wheel and axle, and a wedge is used to open the can.
The mechanical efficiency of most compound machines is low, since they have more
moving parts than simple machines do, and so there is more friction to overcome.
Information Posted on 12/10/05
Notes for Chapter 7 – Forces in Fluids
Section 1 – Fluids and Pressure –
A fluid is any material that can flow and that takes the shape of a container. Because of
these properties, both liquids and gases and considered to be fluids. Fluids can flow
because the particles that make them up are spread farther apart than in a solid, so they
can move, or flow around one another. This also makes them take the shape of whatever
container they are put in.
Because of this, fluids exert pressure. A tire stays inflated because the millions of gas
particles are both striking each other and the inner walls of the tire. Together, these
collisions create a force on the tire. The amount of force exerted on a given area (in this
case, the inner tire), is called pressure.
The SI unit for pressure is the pascal. One pascal (1 Pa) is the force of one Newton
exerted over an area of one square meter (1 N/m2). Pressure can be calculated by using
the equation:
force
pressure = ----------area
Atmospheric pressure is the pressure caused by the weight of the atmosphere. It is
exerted on everything on Earth. Every square centimeter of your body feels about 10 N
(2 lbs) of weight caused by this atmospheric pressure. However, like air inside a balloon
(see Fig. 2, p. 181), fluids in your body also exert a pressure to counteract atmospheric
pressure – this is why you don’t feel the crushing weight of the air above. Atmospheric
pressure varies with altitude (how high or low you are with respect to Earth’s surface.
Pressure is greatest at sea level, because the full pressure of the atmosphere is being
exerted on you. High atop a mountain, however, the atmospheric pressure is about 1/3 of
that at sea level (see Fig. 3, p. 182). If you travel to higher or lower points in the
atmosphere, the fluids in your body have to adjust to maintain equal pressure (this is what
happens anytime your ears ‘pop’ – because of pressure changes in pockets of air behind
your eardrums). Atmospheric density also decreases as you increase in altitude. This is
why airplanes have to be pressurized so that there is enough oxygen for people to breath
comfortably. High above sea level, oxygen molecules are farther apart, therefore the
need to provide a means for additional oxygen, whether they be on an airplane, or
climbing a mountain.
Water is a fluid, and so exerts pressure just like the atmosphere does. A diver, then,
experiences not only the force of water pressure, but also atmospheric pressure, since
atmospheric pressure presses down on the water that they are swimming in. Water
pressure does not depend on the amount of fluid present – a swimmer would feel the
same pressure swimming 3 m below a small pond and at 3 m below the ocean surface.
Because water is about 1,000 times denser than air, water exerts more pressure than air
does (see Fig. 4, p. 183).
Fluids flow from areas of high pressure to areas of low pressure. We see this principle at
work, for example, in the action of drinking from a straw – pressure is reduced in the
straw when air is removed when you drink from it. The outside pressure forces the liquid
up the straw and into your mouth. Also, when breathing, the pressure in your lungs
becomes lower than the pressure outside your lungs. Air then flows into your lungs (see
Fig. 5, p. 184). A tornado’s air pressure is actually very low. They tend to suck up
material around them, however – the pressure outside the tornado is higher than inside,
and so objects are pushed into the tornado.
Section 2 – Buoyant Force –
Buoyant force is the upward force that fluids exert on all matter. When an object is
immersed in water, the water exerts fluid pressure on all sides of the object. The pressure
exerted horizontally on either side of the object is equal – therefore the pressures cancel
out one another. The only fluid pressures affecting the net force on the object are at the
top and bottom (see Fig. 1, p. 186). Since pressure increases as depth of water increases,
the pressure at the bottom of the object is greater than the pressure at the top. The water,
therefore, exerts a net upward force – buoyant force – on the object.
The ancient Greek mathematician Archimedes discovered how one could determine
buoyant force. Archimedes’ principle states that the buoyant force on an object in a
fluid is an upward force equal to the weight of the fluid that the object takes the place of,
or displaces.
An object in a fluid will sink if its weight is greater than the buoyant force (the weight of
the fluid it displaces). An object floats only when the buoyant force on the object is equal
to the object’s weight (see Fig. 2, p. 187). An object sinks when its density is greater
than that of water, which has a density of 1 g / cm3, and an object floats if its density is
less than one. We use the following equation to compute the density of an object or
substance:
mass
density = -------------volume
Most substances are denser than air, so few substances float in the air. One exception is
the element helium, which has a density one-seventh that of air. A given volume of
helium displaces an equal volume of air that is much heavier than itself – so helium floats
in air, which is why it’s used in parade balloons (Fig. 3, p. 188).
We can change the overall density of an object by changing its shape. For example, ships
are built with a hollow shape. So although the amount of steel in the shape is the same as
there would be if the steel was in the shape of a block, the hollow shape increases the
volume the ship, so following the above equation, increasing the ship’s volume leads to a
decrease in its density. Thus, ships made of steel float because their overall density is
less than that of water (see Fig. 4, p. 189).
We can also change an object’s mass to control its overall density. As Figure 5, p. 190
shows, a submarine can stay afloat when its ballast tanks are filled mostly with air (which
is about 1000 times less dense than water). When the crew want the submarine to
descend to a certain depth, the ballast tank vent holes are opened, and water fills them.
The submarine’s mass is increased, although its volume stays the same. From the above
equation, the submarine’s density in this situation is increased, and so it sinks down into
the water. When the submarine rises toward the water’s surface, compressed air pumped
into the tanks forces the water out, thereby decreasing the mass, and decreasing the
density – which allows the vessel to rise.
Similar to a submarine, certain fish adjust their overall density to stay at a certain depth in
the water. Fish that have a swim bladder (Fig. 6, p. 191) can fill it with gases, thereby
inflating the bladder. This increases the fish’s volume and thereby decreases the fish’s
overall density, which keeps the fish from sinking in the water.
Section 3 – Fluids and Motion –
Bernoulli’s principle states that as the speed of a moving fluid increases, the fluid’s
pressure decreases. In the class demonstration of this principle with the two pieces of
paper, air speed between the two sheets increased when air was blown between them.
Because the air speed was increased, the pressure between the two sheets decreased.
Thus, the higher pressure on the outside of the sheets pushed them together.
There are many other examples that demonstrate Bernoulli’s principle. Look at Figure 1,
p. 192, which shows a tennis ball attached to a string and swung into a stream of water.
You might think that the ball would be pushed out of the water, but instead it is held
there. The reason for this is that the water is moving faster than the air around it, so the
water has a lower pressure than the surrounding air. The higher air pressure pushes the
ball into the area of lower pressure – the water stream.
Airplanes and birds in flight also demonstrate Bernoulli’s principle. As Figure 2,p. 193
shows, since air is moving slower beneath the plane’s wing, the greater pressure below
the wing exerts an upward force. This upward force is called lift, and it pushes the wings
(and the rest of the aircraft or bird) upward against the downward pull of gravity. While
the amount of lift created by a plane’s wing is determined partly by the speed at which air
travels around the wing, the speed of a plan is determined mostly by its thrust – the
forward force produced by the plane’s engine.
Lift also depends partly on the size of a plane’s wings. As Figure 3, p. 194 shows, the
large thrust of the jet pushes it through the air at great speeds. But the glider doesn’t have
an engine. It does, however, have a large wing area. So its large wings create the lift it
needs to stay in the air. A similar situation exists with bird wings. A bird with small
wings has to flap them quickly to stay in the air. But a hawk only flaps them
occasionally because it has large wings. When fully extended, the hawk can fly with
little effort, gliding on wind currents and generating the lift needed to stay in the air.
Bernoulli’s principle even extends into the world of sports, as Figure 4, p. 195 shows in
the case of the infamous pitch known as the screwball. Because the ball is spinning
clockwise, the direction of air flow on the left is moving in the opposite direction, and so
moves slower than the air on the right of the ball, which is moving in the same direction
of the ball’s spin. So the lower air speed on the left side of the ball translates into greater
pressure on the left side of the ball, causing it to be pushed to the right side.
If you’ve ever walked against the force of a strong wind, you probably felt like you were
being pushed backward. Fluids (in this case air) exert a force that opposes the motion of
objects moving through the fluids. Drag is the force that opposes or restricts motion in a
fluid. Drag can also work against the forward motion of a plane or bird in flight. Drag is
usually caused by an irregular flow or air called turbulence. To help deal with
turbulence, plans are equipped with flaps like those shown in Figure 5, p. 196.
Pascal’s principle states that a change in pressure at any point in an enclosed fluid will
be equally transmitted to all parts of that fluid. So if a local water pumping station
increased the water pressure by 20 Pa, the water pressure will be increased the same at a
home 2 km away as that of a store a few blocks away from the station. Hydraulic devices
use Pascal’s principle to move or lift objects. Since liquids cannot be easily squeezed or
compressed into a small space, they are often used in hydraulic devices such as those
found in cranes and forklifts. Hydraulic devices can also multiply forces, as in the case
of car brakes (see Figure 6, p. 197).
Information Posted on 11/29/05
Notes for Chapter 6 – Forces and Motion
Section 1 – Gravity and Motion –
Challenging the Greek philosopher Aristotle, who taught that the rate at which an object
falls was dependent on its mass, the Italian scientist Galileo dropped two cannonballs of
different masses from the Leaning Tower of Pisa in Italy. The result? Disproving
Aristotle, the cannonballs hit the ground at the same time. So objects fall to the ground
at the same rate because the acceleration due to gravity is the same for all objects.
Acceleration is the rate at which velocity changes over time. The acceleration of an
object is the object’s change in velocity divided by the amount of time during which the
change occurs. All objects accelerate toward Earth at a rate of 9.8 meters per second per
second. This rate is written as 9.8 m/s/s, or 9.8 m/s2.
To find the velocity of falling objects, the following equation is used:
v = g x t
In the equation, (v) is the change in velocity, (g) is the acceleration due to gravity, and
(t) is the time the object takes to fall in seconds. See p. 151 for a sample problem.
An object stops accelerating only when the upward force of air resistance is equal to the
downward force of gravity (so the net force is 0 N). The object then falls at a constant
velocity called the terminal velocity.
The force of air resistance pushes up on a falling object, such as apple (Fig. 3, p. 152).
The amount of air resistance acting on an object depends on the size, shape, and speed of
the object. For example, air resistance would affect a flat sheet of paper more than a
crumpled one.
Only objects that are only acted on by the force of gravity can be said to be in a state of
free fall. So this would apply to objects orbiting the Earth, such as the space shuttle (see
Fig. 7, p. 154). As this diagram shows, two motions combine to cause orbiting – the
forward movement, and the object’s free fall toward Earth.
Besides satellites and spacecraft, many natural objects are in orbit in the universe, such as
moons around planets and planets around stars. Any object in circular motion is
constantly changing direction, and to do this an unbalanced force is needed. The
unbalanced force that causes objects to move in a circular (or near circular) path is called
centripetal force. Centripetal means “toward the center.”
Projectile motion is the curved path that an object follows when thrown, launched, or
otherwise projected near the surface of the Earth. Projectile motion has two components
– horizontal motion and vertical motion. The two components have no effect on each
other. When the two motions are combined, they form a curved path, as shown in Fig. 9,
p. 155.
Section 2 – Newton’s Laws of Motion –
Newton’s First Law of Motion:
An object at rest remains at rest, and an object in motion remains in motion at constant
speed and in a straight line unless acted on by an unbalanced force. Any object that is
not moving is said to be at rest. A golf ball sitting on a tee will remain there unless acted
on by an unbalanced force (the golf club). The second part of the law states that a
moving object will continue to move forever unless acted on by an unbalanced force that
will stop its motion. That unbalanced force is friction. Friction will, for example, make a
car slow down, and will cause a rolling ball to slow down and stop.
The tendency of objects to remain in motion or stay at rest is call inertia. The more
massive an object the more inertia it has. Thus the inertia of an object is related to its
mass (see Fig. 3, p. 160).
Newton’s Second Law of Motion:
The acceleration of an object depends on the mass of the object and the amount of
force applied. In the first part, acceleration depends on mass. For example, you only
have to exert a small force on an empty cart to accelerate it. But the same amount of
force will not accelerate a full cart as much as the empty cart – you’d have to apply more
force to accelerate the full cart (see Fig. 4, p. 161). So Newton’s second law of motion
shows how force, mass and acceleration are related. The equation that is used to express
this relationship is:
F = m x a or
F
a = -------m
Newton’s Third Law of Motion:
Whenever one object exerts a force on a second object, the second object exerts an
equal and opposite force on the first.
So the third law of motion states that for every action, there is an equal and opposite
reaction - every force must have an equal and opposite reaction. So forces act in pairs –
action and reaction force pairs are present even when no motion is taking place, as in the
force you exert on a chair when you sit on it. Your weight pushing down on the chair is
the action force. The reaction force is the force exerted by the chair that pushes up on
your body. The force is equal to your weight. For more examples of this principle, see
Figures 6 & 7, p. 163-164.
Section 3: Momentum – The momentum of an object depends on the object’s mass and
velocity. The more momentum an object has, the harder it is to stop the object or change
its direction. For example, imagine a compact car and a large truck traveling with the
same velocity. The drivers of both vehicles put on the brakes at the same time. Which
vehicle will stop first? Because the truck has more mass and more momentum than the
car has, a larger force is needed to stop the truck. So the compact car will stop first.
Momentum (p) can be calculated with the equation:
p=mxv
In the equation, m is the mass of an object in kilograms, and v is the object’s velocity in
meters per second (see the example at the top of p. 167).
The Law of Conservation of Momentum states that any time objects collide, the total
amount of momentum stays the same. Look at Figure 2 on p. 167. The white cue ball
had a certain amount of momentum before the collision. During the collision, the cue
ball’s momentum was transferred to the red billiard ball. After the collision, the billiard
ball moved away with the same amount of momentum the cue ball had. So this example
shows how momentum is conserved, or stays the same.
Sometimes objects stick together after a collision – like the football players on p. 168.
After two objects stick together, they move as one object. The mass of the combined
object is equal to masses of the two objects added together. In a head on collision, the
combined objects move in the direction of the object that had the greater momentum
before the collision. But together, the objects have a different velocity than the velocity
of either object before the collision. The objects have a different velocity because
momentum is conserved and depends on mass and velocity. When mass changes,
velocity must change too.
Sometimes objects bounce off of each other in collisions - for example, the bowling ball
and pins shown in Figure 3, p. 168. During these types of collisions, momentum is
usually transferred from one object to another object. This transfer of momentum causes
the objects to move in different directions at different speeds. The total momentum,
however, of all the objects will remain the same before and after the collision.
Conservation of momentum can be explained by Newton’s third law of motion, as Figure
4, p. 169 shows. The action force (the cue ball hitting the billiard ball with a certain
amount of force) makes the billiard ball begin moving, and the reaction force (the equal
but opposite force exerted by the billiard ball on the cue ball) stops the ball’s motion.
Information Posted on 11/14/05
Notes for Chapter 5 – Matter in Motion
Section 1 – Measuring Motion –
We often think of the motion of an object as something easy to detect – you just watch
the object. But you are actually watching the object in relation to another object that
stays in place. When an object changes position over time relative to a reference point,
that object is in motion.
Speed is the distance traveled by an object divided by the time taken to travel that
distance. To determine average speed, the following equation is used:
Average speed =
Total distance
--------------------Total time
The SI (Metric) unit for speed is meters per second (m/s). However, kilometers per hour
(km/h), feet per second (ft/s), and miles per hour (mi/h) are other units commonly used to
express speed.
Velocity is the speed of an object in a particular direction. It is important to remember
that speed and velocity are not the same. For example, two birds might leave a tree at the
same speed – 10 km/h for 5 minutes, 12 km/h for 8 minutes, and 5km/h for 10 minutes,
but not end up in the same place. Why? They went in different directions. So the speeds
were the same, but they had different velocities.
Velocity must include a reference direction. To say that an airplane’s velocity is 600
km/h would not be correct. You would have to include a reference direction, such as 600
km/h south. NOTE: See Fig. 3 & 4 on p. 121.
Acceleration is the rate at which velocity changes. Velocity changes if speed changes, if
direction changes, or if both change. So, an object accelerates if its speed, its direction,
or both change.
•
•
If an object is speeding up, we say that the object has a positive acceleration.
If an object is slowing down, we say that the object has a negative acceleration.
•
You can find the average acceleration by using the equation:
Average acceleration =
Final velocity – starting velocity
---------------------------------------------Time it takes to change velocity
*The SI (Metric) units for Acceleration:
• Velocity is measured in meters per second (m/s)
• Time is measured in seconds (s)
Average acceleration =
•
Final velocity – starting velocity
---------------------------------------------Time it takes to change velocity
m/s
----s
The units are meters per second ÷ second which is meters per second per second
or meters per second squared.
•
•
The SI unit meters per second per second mean that an object speeds up by a
certain velocity every second.
Example: If a cyclist speeds up to 5m/s2 from 1 m/s2 in 4 s, they have increased
their speed by 1 m/s every second, or have an acceleration of 1 m/s2.
NOTE: See Fig. 5 on p. 122.
Section 2 – What is a Force?
• In science, a force is simply a push or a pull. Put in a more detailed way, a force
is a push or pull exerted on an object in order to change the motion of the object.
• Forces cause changes in three things:
–
–
–
Shape
Direction
Speed (causing acceleration or deceleration)
The SI unit for force is the Newton (N). This unit is names after the famous scientist Sir
Issac Newton.
Usually, more than one force is acting on an object. The net force is the combination of
all forces acting on an object. To determine the net force on an object if all forces act in
the same direction, you add the two forces together. If one person pushes with a force of
25N and the other pulls in the same direction with a force of 20N, then you simply add
these numbers to find the net force (25N + 20N = 45N). See Fig. 3, p. 125. If the two
forces are opposing one another, as the two dogs pulling on a rope in Fig. 4 p. 126, then
you subtract the two opposing forces: 12N – 10N = 2N.
Forces act on all objects at all times. They can be balanced or unbalanced. Balanced
forces will not cause a change in the motion of a moving object. Why? Because when
the forces on an object produce a net force of 0N, then the forces are said to be balanced
(See Fig. 5, p. 126). On the other hand, when the net force on an object is not 0N, then
the forces on an object are unbalanced (See Fig. 127, p. 127).
Section 3 – Friction- A Force That Opposes Motion
Friction is a force that opposes motion between two surfaces that are in contact.
Basically, when the microscopic hills and valleys of one surface stick to the hills and
valleys of another surface, friction is created.
Rougher surfaces have more microscopic hills and valleys than smooth surfaces do. So
the rougher the surface is, the greater the friction.
There are two types of friction. If you slide a stack of books across a table, you are
witnessing kinetic friction. Kinetic means “moving,” so the amount of kinetic friction
between two surfaces depends in part on how the surfaces move. Surfaces can slide past
each other, or a surface can roll over another surface. Usually, the force of sliding kinetic
friction is greater than the force of rolling kinetic friction – which is why it’s easier to
move a piece of heavy furniture on wheels rather than just sliding it across the floor (See
Fig. 3, p. 130). When a force is applied to an object but does not cause the object to
move, static friction occurs. Static means “not moving.” Suppose you try to push a stack
of heavy books across a table with your finger. The books don’t move because the force
of static friction balances the force applied by your finger (See Fig. 4, p. 131).
Friction can be both helpful and harmful. Without friction, the tires on your car could
not push against the ground to move the car forward, and the brakes could not stop the
car – so this is a good example of friction being helpful. On the other hand, friction
between moving engine parts causes the parts to eventually wear down, so this is an
example of friction being harmful.
Friction can be reduced. You can reduce the amount of friction by using lubricants such
as oil, wax, or grease. Friction can also be reduced by switching from sliding kinetic
friction to rolling kinetic friction (as in the example of moving heavy furniture). Still
another way to reduce friction is to make surfaces that rub against each other more
smoothly. Sanding a park bench will make the bench smoother. Therefore, the bench
will be more comfortable now to sit on because the friction between your leg and the
bench is reduced.
Friction can be increased. One way to make friction increase is to make surfaces
rougher. For example, sand scattered on icy roads keeps cars from skidding. You often
see baseball players using textured batting gloves to increase friction between the bat and
their hands so that the bat won’t fly out of their hands when they swing.
Section 4 – Gravity: A Force of Attraction
Gravity is a force of attraction between objects that is due to their masses. All matter has
mass, and gravity is a result of mass. Therefore, all matter is affected by gravity. Put
another way, all objects experience an attraction toward all other objects. You usually
don’t notice objects moving towards each other because the mass of most objects is too
small to cause a force large enough to move objects toward each other. Of course, we are
all familiar with one object massive enough to cause a noticeable attraction on other
objects – Earth. Earth has a huge gravitational force, and pulls everything on its surface
toward the center. Because of this force, books, tables and chairs stay in one place, and
dropped objects fall to Earth rather than moving together or toward you.
Sir Isaac Newton realized that the force of gravity could answer two basic questions that
people had asked for thousands of years: Why do objects fall toward Earth, and what
keeps the planets moving in the sky? Newton summarized his ideas in a law now known
as the law of universal gravitation, which states that all objects in the universe attract
each other through gravitational force.
The Law of Universal Gravitation can be broken into two parts:
Part 1: Gravitational force increases as mass increases – Astronauts bounce when they
walk on the moon? Why? Because the moon’s mass is about 1/6 that of the Earth. So
the moon has less mass, which causes less of a gravitational pull on the astronaut’s body.
Gravitational force is small between objects that have small masses, and gravitational
force is large when the mass of one of both objects is large (see Fig. 3, p. 136).
Part 2: Gravitational force decreases as distance increases – If you jump up, you are
immediately pulled down to the surface of the Earth by the Earth’s gravitational force.
On the other hand, the Sun is more than 300,000 times more massive than Earth.
However, the Sun’s gravitational force doesn’t affect you more than Earth’s because the
Sun is so far away (93 million miles)! If, however, you could stand on the Sun, you
would find it impossible to move, because the gravitational force acting on your body
would be so great that you could not move any part of your body. So, in summary
gravitational force is strong when the distance between two objects is small. If, however,
the distance between two objects increases, then the gravitational force pulling them
together decreases rapidly.
Weight and mass, while related to one another, is not the same thing! Mass is the
amount of matter in an object. It does not change if you change your location, even if it’s
on another planet or moon. Weight, however, is a measure of the gravitational force on
an object. So an astronaut on the moon will weigh about 1/6th his or her weight on Earth,
but his or her mass remains constant – it doesn’t change (see Fig. 6, p. 138). We learned
in an earlier section that the SI unit of force is a newton (N). Since gravity is a force, it
too is measured in newtons. The SI unit for mass is kilograms (kg). Mass is also
measured in grams (g) and milligrams (mg) as well.
Notes for Quarter I
Information Posted on 10/19/05
Notes for Chapter 4 – Elements, Compounds, and Mixtures
Section 1 – Elements –
• An element is a pure substance that cannot be separated into simpler substances
by physical or chemical means.
• A pure substance is a substance in which there is only one type of particle.
ex. – a sample of gold is a pure substance, since every atom in the sample is like
every other gold atom.
Properties of Elements • Each element has its own characteristic properties
• Physical properties would include such things as melting point, density, and
boiling point.
• Chemical properties would include reactivity with another substance,
flammability, etc.
• Elements may share properties with other elements (ex.-helium & krypton are
both unreactive gases but have different densities)
Identifying Elements by Their Properties –
• Each element can be identified by its own unique properties
• Examples of element identification by chemical properties – zinc is reactive with
acid; hydrogen and carbon are flammable
• Examples of element identification by physical properties – sulfur is yellow
(color), aluminum is malleable
Classifying Elements by Their Properties –
• Elements are grouped into categories by the properties they share
• Elements are classified as metals, nonmetals, or metalloids
Metals –
• Metals are shiny, and conduct heat and electric current
• Metals are also malleable (can be hammered into thin sheets), and ductile (can be
drawn through a wire)
• Examples of metals are lead, copper, and tin.
Nonmetals • Nonmetals do not conduct heat or electric current, and solid nonmetals are dull
(not shiny) in appearance, and may be brittle and unmalleable.
• Examples of nonmetals are iodine, sulfur, neon, and xenon.
Metalloids –
• Metalloids have properties of both metals and nonmetals – some are shiny, some
are dull.
• Metalloids are somewhat malleable and ductile.
• Some metalloids conduct heat and electric current well, but some do not.
• Examples of metalloids include boron, antimony, and silicon
Section 2 – Compounds – A compound is a pure substance composed of two or more
elements that are chemically combined.
• There are many examples of compounds that we encounter every day. The
elements sodium and chlorine, for example, combine to form table salt – sodium
chloride. Iron and oxygen combine to form rust – iron oxide.
• Compounds can be identified by their physical properties (melting point, density,
color, etc.), as well as chemical properties (reactivity with acid, reactivity to light,
etc.).
• It is important to understand that a compound has properties that differ from
those of the elements that form it. For example, take sodium chloride. When we
look at the elements that form it, we have sodium – a soft, silvery-white metal that
reacts explosively if brought into contact with water. Chlorine is a poisonous,
green gas. But when we chemically combine these two elements, a harmless
compound is formed that has unique properties – properties that differ from the
elements that formed it.
• Some compounds can be broken down into their elements by chemical changes
by applying heat or electric current. For example, you can heat the compound
mercury oxide, and a chemical change occurs, causing it to separate into its
component elements mercury and oxygen. Other compounds break down to form
simpler compounds instead of elements. For example, carbonic acid (found in
soda) breaks down into the compounds carbon dioxide and water.
• Compounds are found in nature, but often the compounds found in nature are not
the raw materials needed by industry. Ammonia is a common compound used in
industry, used to make fertilizers. Combining the elements nitrogen and hydrogen
makes ammonia. Many examples of compounds can be found in the natural
world. Proteins, for example, are compounds found in all living things. Carbon
dioxide is another important compound in nature, and is of central importance in
the process of photosynthesis – the means by which plants make food and release
oxygen into the atmosphere.
Section 3 – Mixtures – A mixture is a combination of two or more substances that are
not chemically combined. So when two or more materials are put together they form a
mixture if they do not react chemically to form a compound. For example, cheese and
tomato sauce do not react when they are used to make a pizza – so a pizza is a mixture.
A salad (with lettuce, tomatoes, onions, etc.) would be another common example of a
mixture.
• You can sometimes easily separate mixtures through physical methods – for
example, taking mushrooms off of a pizza. Other mixtures are not so easily
separated. For example, you can’t pick the salt out of a saltwater mixture.
Although you could heat the saltwater mixture, evaporating the water and leaving
the salt behind (see p. 99 for other ways to separate mixtures).
• Whereas a compound is made of elements in a specific mass ratio, the
components of a mixture do not need to be mixed in a definite ratio. For example,
granite is a mixture made of three minerals, feldspar, mica, and quartz. Even
though the proportions of these minerals may change, this combination of
minerals is always a mixture called granite.
A solution is a mixture that appears to be a single substance. It is composed of particles
of two or more substances that are distributed evenly among each other. Solutions have
the same appearance and properties throughout the mixture.
• The process in which particles of substances separate and spread evenly
throughout a mixture is called dissolving. The solute is the substance that is
being dissolved, and the solvent is the substance in which the solute is dissolved.
For example, salt water is a solution. Salt is soluble in water, meaning salt
dissolves in water. So salt would be the solute, and water the solvent.
• Solutions can be liquids (ex. Gasoline, soft drinks, many cleaning supplies, etc.).
However, solutions may also be gases, such as air, or even solids, such as steel.
Alloys are solid solutions of metals or nonmetals dissolved in metals (brass is an
alloy of the metal zinc dissolved in copper).
• Particles in solutions are so small that they never settle out. They also cannot be
removed by filtering (see, for example, figure 4 on p. 101).
• A measure of the amount of solute dissolved in a solvent is concentration.
Concentration is expressed in grams of solute per milliliter of solvent (g/mL).
Solutions can be described as being concentrated or dilute. These terms,
however, do not tell you the amount of solute that is being dissolved. In Figure 5
on p. 102, the two solutions have the same amount of solvent, but one has less
solute than the other. Accordingly, one is dilute, and the other concentrated. So a
dilute solution will contain less solute, and a concentrated solution will contain
more solute.
• The solubility of a solute is the ability of the solute to dissolve in a solvent at a
certain temperature. Most solids are more soluble in liquids at higher
temperatures, but gases become less soluble in liquids as the temperature is raised.
• A suspension is a mixture in which particles of a material are dispersed
throughout a liquid or gas but are large enough that they settle out.
• Some mixtures have properties between those of solutions and suspensions –
these mixtures are known as colloids. A colloid is a mixture in which the
particles are dispersed throughout but are not heavy enough to settle out. So the
particles in a colloid are small and well mixed. Milk, mayonnaise, and whipped
cream are all examples of colloids.
Information Posted on 10/10/05
Notes for Chapter 3 – States of Matter
Section 1 – Three States of Matter – The three states of matter that we will focus on are
solid, liquid, and gas (a fourth state, plasma, occurs at very high temperatures, such as
those found within stars like the Sun. Electrons are stripped away from their parent
atoms in a plasma). States of matter are the physical forms in which a substance can
exist. Water, for example, commonly exists in three states: solid (ice), liquid (water), and
gas (steam). Solids have a definite shape and volume. There are two kinds of solids –
crystalline and amorphous. Crystalline solids have a very orderly, 3-D arrangement of
particles (ex. Diamonds, iron and ice). Amorphous solids are made of particles that do
not have a special arrangement (ex. Glass, rubber and wax). Liquids have a definite
volume, but they take the shape of the container that they are in. A special property of
liquids is surface tension – a force that acts on the particles at the surface of a liquid.
This is what causes some liquids to form spherical drops like dew on grass. Another
important property of liquids is viscosity – a liquid’s resistance to flow. Generally, the
stronger the attraction between molecules that make up a liquid, the more viscous it is
(honey has a higher viscosity than water). Gas is the state of matter that has no definite
shape or volume. The particles of a gas move quickly, so they can break away
completely from one another. The particles of a gas move quickly, so they can break
away completely from one another. So the particles of a gas have less attraction between
them than do particles of the same substance in the solid or liquid state.
Section 2 – Behavior of Gases – Gases behave differently from solids or liquids, in that
the particles that make up gases are spaced widely apart. To understand gas behavior, we
have to understand the relationship between temperature, volume, and pressure.
Temperature is a measure of how fast the particles in an object are moving. If you take a
balloon and put it outside on a hot day, you will notice that the balloon will expand. This
is because the heat will causes the particles in the balloon to move faster – they collide
with the inner walls of the balloon with greater force, causing the balloon to grow in size.
But on a cool day, the particles of gas in the balloon have less energy, and so do not push
as hard on the walls of the balloon. Volume is the amount of space that an object takes
up. Because the particles of a gas are spread out, the volume of any gas depends on the
container that the gas is in. Gas particles can be compressed much more easily than
particles of a liquid, which is why you can bend and twist a balloon filled with air, but
you cannot do so with one filled with water – the balloon would break! Pressure is the
amount of force exerted on a given area of surface. You can also think of it as the
number of times the particles of a gas hit the inside of their container. A basketball has a
higher pressure than a beach ball because inside the basketball, there are more particles of
gas in it, and they are closer. The particles collide with the inside of the ball at a faster
rate. The beach ball has a lower pressure, on the other hand, because there are fewer
particles of gas, and they are farther apart. The particles in the beach ball collide with
inside of the ball at a slower rate, thereby leading to lower pressure.
Robert Boyle, a 17th century Irish chemist, described the relationship between the volume
and pressure of a gas. Boyle’ Law states that the volume of a gas is inversely
proportional to the pressure of a gas when temperature is constant (see Fig. 3 on p. 72).
So as pressure increases, the volume of a gas decreases, and as pressure is decreased, the
volume of a gas increases. Charles’ Law states that the volume of a gas is directly
proportional to the temperature of a gas when pressure is constant (see Fig. 4 on p. 73).
So decreasing the temperature of a gas causes the particles to move more slowly, thereby
leading to a decrease in volume. Increasing the temperature of the gas causes particles to
move more quickly, thereby leading to a greater volume.
Section 3 – Changes of State – A change of state is the change of a substance from one
physical form to another (ice to liquid to gas, for example). All changes of state are
physical changes – the identity, or chemical composition never changes. The particles of
a substance move differently, and have different amounts of energy, depending on what
state it is in. Particles in liquid water have more energy than particles in ice, and so move
faster. Particles in steam have more energy still, and so move even faster. In order to
change a substance from one state to another, energy must be added or removed.
Melting is the change of state from solid to liquid. By adding energy to an ice cube, for
example, the temperature of the ice cube is increased, and the ice particles move faster.
When a certain temperature is reached, the ice will melt. A substance’s melting point is
that temperature at which the substance changes from a solid to a liquid. For a solid such
as ice to melt, particles must overcome some of their attractions to each other. When a
solid is at its melting point, and energy added to it is used to overcome the attractions that
hold the particles in place. So melting is an endothermic reaction – because energy is
gained by the substance as it changes state.
Freezing is the change of state from a liquid to a solid. The temperature at which a liquid
changes into a solid is the liquid’s freezing point. Freezing is the reverse process of
melting; thus freezing and melting occur at the same temperature (see Fig. 3, p. 75). So if
energy is added at 0 degrees Celsius, the ice will melt, because any energy added goes
into breaking the bonds of attraction of the particles. If energy is removed at 0 degrees
Celsius, the liquid will freeze, because removing energy will cause the particles to begin
locking into place. So freezing is an exothermic reaction, because energy is removed
from the substance as it changes state.
Evaporation is the change of a liquid to a gas. It is possible for evaporation to occur at
the surface of a liquid that is below its boiling point (leave a glass of water out and you
will notice that the water eventually evaporates). So in the process of evaporation, some
particles at the surface of the liquid move fast enough to break away from the particles
around them and become a gas. Boiling is the change of a liquid to a vapor, or gas,
throughout the liquid (see Fig. 4, p. 76). This is an important point – evaporation occurs
at the surface of a liquid, and boiling occurs throughout the liquid. Boiling occurs when
the pressure inside the bubbles (called vapor pressure) equals the outside pressure on the
bubbles (called atmospheric pressure). The temperature at which a liquid boils is its
boiling point. Water boils at 100 degrees Celsius at sea level. However, in Denver,
about 1.6 km above sea level, the atmospheric pressure is lower, since there is less air
above you. Therefore, since there is less atmospheric pressure, the boiling point of water
will be slightly less – about 95 degrees Celsius.
Condensation is the change of state from a gas to a liquid. Condensation and evaporation
are the reverse of each other. The condensation point of a substance is the temperature at
which the gas becomes a liquid. And the condensation point is the same temperature as
the boiling point at a given pressure. Sublimation is the change of state in which a solid
changes directly into a gas. Dry ice (solid carbon dioxide) is much colder than ice made
from water.
When most substances gain or lose energy, one of two things happen to the substance: its
temperature changes or its state changes. The temperature of a substance, remember, is a
measure of the speed of the particles that make up the substance. So when the
temperature of a substance changes, the speed of the particles also changes. But the
temperature of a substance does not change until the change of state is complete.
For example, the temperature of boiling water stays at 100 degrees Celsius until it has all
evaporated (see Figure 7, p. 79).
Information Posted on 10/02/05
Notes for Chapter 2 – The Properties of Matter
Section 1 – What is Matter? – Matter is anything that has mass and takes up space. The
amount of space take up, or occupied, by an object is known as the object’s volume. The
volume of liquids is most often expressed in the units liters (L) and milliliters (mL). A
graduated cylinder is used to measure the volume of a liquid. Because the surface of a
liquid in any container is curved, to properly read the volume of a liquid in a graduated
cylinder one must look at the bottom of the curve of the meniscus (see p. 39 in text). To
measure the volume of a solid object, the units must be expressed in cubic units (‘cubic’
means having three dimensions). A block of wood, therefore, has three dimensions –
height, width, and length. To find the volume of such an object, one simply multiplies
these three dimensions:
V=lxwxh
So the block of wood might have a length of 14 cm, a width of 5 cm, and a height of 3
cm. Multiplying each of these values gives one the volume of the block of wood. Or, a
graduated cylinder can be used to measure the volume of a solid object by water
displacement – subtracting the water level after an object has been dropped into a
graduated cylinder from the original water level. So, if a graduated cylinder is filled to
the 80 mL line, and a small lead weight is dropped in, the water line might rise to 120
mL. So to find the volume of the lead, one simply subtracts new from original:
V = 120 mL – 80 mL = 40 cubic centimeters
Mass is the amount of matter in an object. Weight is the measure of the gravitational
force exerted on an object. Going to the moon won’t change your mass, but your weight
will be 1/6 of what it is on the Earth, because the moon is 1/6 the size of Earth, and
therefore it will exert a much weaker gravitational pull on an object at its surface. Inertia
is the tendency of an object to resist a change in its motion. So, an object at rest will stay
at rest unless acted on by an outside force, and an object moving will keep moving at the
same speed and direction unless something acts on it to change its speed or direction.
Inertia is the tendency of an object to resist a change in its motion – an object will remain
at rest until something causes it to move. Likewise, a moving object will keep moving at
the same speed and in the same direction unless something acts on it to change its speed
or direction. A more massive object will therefore have greater inertia – pushing a loaded
grocery cart is harder than pushing an empty one.
Section 2 – Physical Properties – A physical property of matter can be observed or
measured without changing the matter’s identity. So, you don’t have to change an
orange’s identity to see its color or measure its volume. Density is one physical property
that describes the relationship between mass and volume, and is found by the formula:
M
D = ______
V
Other physical properties include conductivity, state, solubility, ductility, and
malleability. Physical changes do not produce new substances – crushing a can,
tearing a piece of paper, or melting an ice cream bar are all physical changes – no change
to the original substance (in terms of its chemical makeup) was made – it’s still a can, a
piece of paper, and an ice cream bar – only its size or appearance has changed, or state.
Section 3 – Chemical Properties – Chemical properties describe matter based on its
ability to change into new matter that has different properties. Some chemical properties
include flammability – the ability of a substance to burn - and reactivity – the ability of
two or more substances to combine and form one or more new substances. Burning
paper produces heat and smoke – new substances. An iron nail can react with oxygen in
the air to form iron oxide, or rust – these are examples of chemical changes – when one
or more substances are changed into new substances that have new and different
properties. These two terms are not the same – chemical properties of a substance
describe which chemical changes will and will not occur. Chemical changes are the
process by which substances actually change into new substances. The key question to
ask when trying to determine whether a physical or chemical change has occurred is:
Has a new substance been formed? Did the original composition change? Many physical
changes can be reversed – an ice cube that has melted can be turned back into solid ice by
freezing the liquid water again. However, most chemical changes are not easily reversed.
Information Posted on 9/22/05
Notes for Chapter 1 – The World of Physical Science
Section 1 – Exploring Physical Science – Focuses on the nature of science, which is a
process of gathering knowledge about the natural world. When we make observations of
phenomena, we are prompted to ask questions-the beginning of scientific inquiry.
Physical science is defined as the study of matter and energy. It is divided into the study
of physics and chemistry. Physics looks at energy and the way that energy affects matter.
Chemistry studies the structure and properties of matter and how matter changes.
**Knowledge of physical science is important for many areas of science, such as
geology, meteorology, and biology.
Section 2 – Scientific Methods – Scientific methods are the ways in which scientists
answer questions and solve problems. There are certain steps that scientists use
whenever they are engaged in scientific inquiry. First, an observation is made – this is
any use of the senses to gather information (for example, noting that the sky is blue, or
that a cotton ball feels soft). Scientists are then led to ask questions about their
observations. After gathering preliminary information, scientists are then ready to form a
hypothesis – a possible explanation or answer to a question. A good hypothesis is always
testable. In other words, information can be gathered or an experiment can be designed
to test the hypothesis. Scientists then make a prediction of what they think will happen
before testing the hypothesis. One way to test a hypothesis is to do a controlled
experiment, which compares the results from a control group with the results from
experimental groups. Pieces of information obtained through experimentation are called
data. After testing a hypothesis, it is important to analyze your results by using
calculations, tables, and graphs. Then, after analyzing your results, you should draw
conclusions about whether your hypothesis is supported. Finally, communicating your
results allows others to check or continue your work.
Section 3 – Scientific Models – A model is a representation of an object or system.
There are three kinds of scientific models: Physical, mathematical, and conceptual. A
physical model might be a model space shuttle, or the human eye. Mathematical models
are made up of equations and data, and sometimes use computers. Conceptual models
are often ideas; for example, the big bang theory is a conceptual model. Some models are
smaller than the objects they represent (i.e., globes, solar system models), while other
models are larger than the objects they represent (i.e., molecules, DNA). A scientific
theory is an explanation for many hypothesis and observations. A scientific law
summarizes experimental results and observations. The different between the two is that
a theory is an explanation of why something happened the way it did, and a law is a
statement that tells how things work.
Section 4 – Tools, Measurement, and Safety – A tool is anything that helps you do a
task. Scientists use many tools to help them in their experiments. One way to collect
data is to take measurements. But to do this, you need the proper tools. Stopwatches,
metersticks, and balances are some tools that can be used to make measurements. (See
metric information posted on 9/19 for detailed information on the International System of
Units (SI), or metric system).
Length, volume, mass, and temperature are types of measurement used in science. The
meter is the basic SI unit of length. Mass is the amount of matter in an object, and the
kilogram (kg) is the basic unit for mass. The kilogram is used to describe the mass of
large objects, and the gram is used to measure the mass of smaller objects. Volume is
the amount of space that something occupies. Liquid volume is expressed in liters (L).
Liters are based on the meter. A cubic meter is equal to 1,000 L. Volumes of solid
objects are expressed in cubic meters. If you measure the mass and volume of an object,
you have enough information to measure its density – the amount of matter in a given
volume. Density is called a derived quantity because it is found by combining the two
basic quantities of mass and volume. The equation that relates density to mass and
volume is:
m
D = -------V
The temperature of a substance is a measurement of hot or cold the substance is. Degrees
Fahrenheit and degrees Celsius are often used to describe temperature. The SI unit for
temperature is the Kelvin (K).
You will frequently encounter different safety symbols and rules when engaging in
scientific investigations. Always pay attention to any safety labels on the sides of
chemicals or other equipment – these alert you to what precautions you need to take, such
as wearing goggles or gloves.
Information Posted on 9/19/05
Metric units of measurement
The French Academy of Sciences in the late 1700’s set out to make a simple and reliable
measurement system. Over the next 200 years, the metric system was formed. This
system is now the International System of Units (SI). We will be working with mass,
volume, length, and temperature in our metric studies and conversions.
When working with mass, volume, and length, the “metric staircase” is a helpful visual
tool, especially when doing conversions between units:
If going from a big to a smaller
unit, move decimal point to RIGHT
the number of times you jump
down the staircase.
Ex.- 1 meter = _______millimeters
Meters to millimeters is 3 jumps.
So there’s a decimal point after 1.
km
hm
dkm
m
dm
cm
If going from a smaller to a bigger
unit, move decimal point to LEFT the
number of times you jump up the
staircase.
Ex. – 1 millimeter = _______meters
Millimeters to meters is 3 jumps.
There’s a decimal point after 1.
Move decimal point 3 places to left.
_ _ _ 1.
So 1 millimeter = 0.001 meters
mm
You can remember how the order of the units goes by remembering a simple mnemonic,
or memory aide: King Henry Died Monday Drinking Chocolate Milk.
You can use the metric staircase for meters (length), as we did above, or grams (mass), or
liters (volume). Just be sure to substitute the ‘m’ on the right of the unit with ‘L’ for
liters and ‘g’ for grams (km – kL – kg).
Common SI (Metric) units:
km (kilometer) = 1000 meters
hm (hectometer) = 100 meters
dkm (dekameter) = 10 meters
m (meter) = 1 meter (base unit)
dm (decimeter) = 1/10 of meter
cm (centimeter) = 1/100 of meter
mm (millimeter) = 1/1000 of meter
Information Posted 9/11/05
The Scientific Method is a useful tool for engaging in scientific inquiry. The traditional
steps are:

Observe






Ask a question
Research
Form a hypothesis
Test the hypothesis
Record and analyze data
Form a conclusion
It is also important to repeat your experiment, to exclude the possibility of error in
your experimental setup. You also must communicate your results to the rest of the
scientific community. In this way, you contribute to building up of knowledge and
experience in a particular scientific discipline, and others benefit from your work.
Let’s put these steps into a practical example. You might observe that leaves are
starting to change from green to shades of red, brown, and orange during the fall
season. You then ask a question: “What is causing the leaves to change color during
this time every year?” You then do some background research. You are now in a
position to form a hypothesis: The leaves are changing color as a result of chemical
reactions occurring. You must test this hypothesis, and then record and analyze data
that you obtain. Only now can you form a reasonable conclusion: The leaves are
changing color because of a breakdown in chlorophyll, and secondary chemical
reactions that cause the green color to disappear and hues of red, brown, and orange
to appear in its place.
Experiments are made up of variables, which are factors in an experiment that change.
The Independent Variable (IV) is the factor that the experimenter changes on purpose.
In an experiment that seeks to determine the effect of differing amounts of water on plant
growth, the differing amounts of water would be the IV – the experimenter might give
Plant A 10mL of water, Plant B 20mL, Plant C 30mL, and Plant D 40mL. The factor that
changes as a result of the purposely-changed factor is called the Dependent Variable
(DV). In other words, the experiment changes it. In our plant growth experiment
example, plant growth would be the DV. The independent variable will have different
levels, or ways that the experimenter changes it – this is referred to as Level of
Independent Variable (LIV). In our example, the different ways that he/she changes, or
manipulates the IV are applying 10, 20, 30, and 40mL of water to the different plants.
Variables that do not change in an experiment are called Constants. In our sample
experiment, some constants might be same type of water (distilled), same type of soil,
same type of pot, etc.). An experiment will usually have a Control. A control group is
used for comparison with the experimental groups. So in our example, Plant E might be
the control – it is not given any water. It is a good idea to repeat our experiment, to
reduce the possibility of error. So the Number of Repeated Trials (NRT) refers to the
number of times that each level of independent variable is tested. In our example, Plants
A-D will be given the specified amounts of water (10, 20, 30, and 40mL) of water a total
of three times – so the NRT for this experiment would be 3.
We want to be able to formulate good title and hypothesis statements for our
experiment. When you write an experimental title, you are basically stating what the
effect of the independent variable is on the dependent variable, and you write it in this
format:
The Effect of IV on DV.
So a good title for our plant experiment would be:
The Effect of Amount of Water on Plant Height.
|

|
IV
DV
We then can write a hypothesis statement for our experiment. Hypothesis statements
follow an “If, then” format. Basically, in a hypothesis statement you are making a
prediction about how the dependent variable will change if you make a certain change to
the independent variable: If how you are changing the IV, then how you predict the DV
will change. So a good hypothesis statement for our experiment would be:
If the amount of water given a plant is increased, then the height the plant
will grow will be increased.
There are of course several different hypothesis statements that could be written for an
experiment such as this. Some are written in such a way that a change in magnitude in
the IV reflects a similar magnitude change in the DV. These are in direct proportion:
If IV increases, then DV increases.
If the amount of water given a plant is increased, then the plant height will be increased.
If IV decreases, then DV decreases.
If the amount of water given a plant is decreased, then the plant height will be
decreased.
Others are written in such a way that a change in magnitude in the IV reflects the
opposite change in magnitude in the DV. These are in inverse proportion:
If IV increases, then DV decreases.
If the amount of water given to a plant is increased, then the plant height will be
decreased.
If IV decreases, then DV increases.
If the amount of water given a plant is decreased, then the plant height will be increased.
Let’s take all of this information and place it on an Experimental Design
Diagram:
Experimental Design Frame/Diagram
Title: The Effect of Amount of Water on Plant Height
Hypothesis: If the amount of water given a plant is increased, then the plant
height will be increased.
IV: Amount of water
Levels of IV
(LIV)
# of Repeated
Trials (NRT)
10mL 20mL
3
3
30mL
40mL
3
3
No
water
(control)
DV: Plant height
Constants – C: Type of water, type of soil, type of pot
3