Download Earth`s Interior - Taunton Public Schools

Name ____________________________
Plate Tectonics
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Date ___________________
Class ____________
Section Summary
Earth’s Interior
Guide for Reading
■ How have geologists learned about Earth’s inner structure?
■
What are the characteristics of Earth’s crust, mantle, and core?
Earth’s surface is constantly changing. Earth looks different today from the
way it did millions of years ago. People wonder, “What’s inside Earth?” The
extreme conditions in Earth’s interior prevent exploration far below the
surface. Geologists have used two main types of evidence to learn about
Earth’s interior: direct evidence from rock samples and indirect evidence
from seismic waves.
Rocks from inside Earth give geologists clues about Earth’s structure.
Geologists can make inferences about conditions deep inside Earth where
these rocks formed. Using data from seismic waves produced by
earthquakes, geologists have learned that Earth’s interior is made up
of several layers.
The three main layers of Earth are the crust, the mantle, and the core.
These layers vary greatly in size, composition, temperature, and pressure.
Beneath the surface, the temperature decreases for about 20 meters, then
increases until the center of Earth is reached. Pressure results from a force
pressing on an area. Pressure inside Earth increases as you go deeper.
The crust is the layer of rock that forms Earth’s outer skin. The crust is a
layer of solid rock that includes both dry land and the ocean floor. Oceanic
crust consists mostly of rocks such as basalt, dark rock with a fine texture.
Continental crust, the crust that forms the continents, consists mainly of
rocks such as granite. Granite is a rock that usually is a light color and has a
coarse texture.
Below a boundary 40 kilometers beneath the surface is the solid material
of the mantle, a layer of hot rock. Earth’s mantle is made up of rock that is
very hot, but solid. Scientists divide the mantle into layers based on the
physical characteristics of those layers. The uppermost part of the mantle
and the crust together form a rigid layer called the lithosphere. Below the
lithosphere is a soft layer called the asthenosphere. Beneath the
asthenosphere, the mantle is solid. This solid material, called the lower
mantle, extends all the way to Earth’s core.
The core is made mostly of the metals iron and nickel. It consists of
two parts—a liquid outer core and a solid inner core. The outer core is a
layer of molten metal that surrounds the inner core. The inner core is a dense
ball of solid metal.
Scientists think that movements in the liquid outer core create Earth’s
magnetic field. Because Earth has a magnetic field, the planet acts like a giant
bar magnet.
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Name ____________________________
Plate Tectonics
■
Date ___________________
Class ____________
Section Summary
Convection and the Mantle
Guide for Reading
■ How is heat transferred?
■
What causes convection currents?
■
What causes convection currents in Earth’s mantle?
The movement of energy from a warmer object to a cooler object is called
heat transfer. Heat is always transferred from a warmer substance to a cooler
substance. There are three types of heat transfer: radiation, conduction,
and convection.
The transfer of energy through empty space is called radiation. Heat
transfer by radiation takes place with no direct contact between a heat source
and an object. For example, radiation enables sunlight to warm Earth’s
surface.
Heat transfer by direct contact of particles of matter is called conduction.
In conduction, the heated particles of a substance transfer heat to other
particles through direct contact. An example is when a spoon heats up in
a hot pot of soup.
The transfer of heat by the movement of a heated fluid is called
convection. Fluids include liquids and gases. During convection, heated
particles of a fluid begin to flow, transferring heat energy from one part of
the fluid to another.
Heat transfer by convection is caused by differences in temperature and
density within a fluid. Density is a measure of how much mass there is in a
volume of a substance. When a liquid or gas is heated, the particles move
faster. As they move faster, they spread apart. Because the particles of the
heated fluid are farther apart, they occupy more space. The fluid’s density
decreases. But when a fluid cools, the particles move closer together and
density increases.
An example of convection occurs in heating a pot of soup on a stove. As
soup at the bottom of the pot gets hot, it expands and becomes less dense. The
warm, less dense soup moves upward, floating over cooler, denser soup. At
the surface, the warm soup spreads out and cools, becoming denser. Then
gravity pulls this cooler, denser soup down to the bottom, where it is heated
again and begins to rise. This flow that transfers heat within a fluid is called a
convection current. The heating and cooling of the fluid, changes in the
fluid’s density, and the force of gravity combine to set convection currents
in motion. Convection currents continue as long as heat is added to the fluid.
Convection currents flow in the asthenosphere. The heat source for these
currents is heat from Earth’s core and from the mantle itself. Hot columns of
mantle material rise slowly. At the top of the asthenosphere, the hot material
spreads out and pushes the cooler material out of the way. This cooler
material sinks back into the asthenosphere. Convection currents like these
have been moving inside Earth for more than four billion years!
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Name ____________________________
Plate Tectonics
■
Date ___________________
Class ____________
Section Summary
Drifting Continents
Guide for Reading
■ What was Alfred Wegener’s hypothesis about the continents?
What evidence supported Wegener’s hypothesis?
■
Why was Alfred Wegener’s theory rejected by most scientists of his day?
In 1910, a young German scientist named Alfred Wegener became curious
about why the coasts of several continents matched so well, like the pieces
of a jigsaw puzzle. He formed a hypothesis that Earth’s continents had
moved! Wegener’s hypothesis was that all the continents had once been
joined together in a single landmass and have since drifted apart. He
named this supercontinent Pangaea, meaning “all lands.” According to
Wegener, Pangaea existed about 300 million years ago. Over tens of millions
of years, Pangaea began to break apart. The pieces of Pangaea slowly moved
toward their present-day locations, becoming the continents of today. The
idea that the continents slowly moved over Earth’s surface became known
as continental drift. In a book called The Origin of Continents and Oceans,
Wegener presented his evidence. Wegener gathered evidence from
different scientific fields to support his ideas about continental drift. He
studied land features, fossils, and evidence of climate change.
Mountain ranges and other landforms provided evidence for continental
drift. For example, Wegener noticed that when he pieced together maps of
Africa and South America, a mountain range running from east to west in
South Africa lines up with a range in Argentina. Also, European coal fields
match up with coal fields in North America.
Fossils also provided evidence to support Wegener’s theory. A fossil is
any trace of an ancient organism preserved in rock. The fossils of the reptiles
Mesosaurus and Lystrosaurus and a fernlike plant called Glossopteris have
been found on widely separated landmasses. This convinced Wegener that
the continents had once been united.
Wegener used evidence from climate change to further support his
theory. For example, an island in the Arctic Ocean contains fossils of tropical
plants. According to Wegener, the island once must have been located close
to the equator. Wegener also pointed to scratches on rocks made by glaciers.
These scratches show that places with mild climates today once had climates
cold enough for glaciers to form. According to Wegener’s theory, Earth’s
climate has not changed. Instead, the positions of the continents have
changed.
Wegener also attempted to explain how the drift of continents took place.
Unfortunately, Wegener could not provide a satisfactory explanation for
the force that pushes or pulls the continents. Because he could not identify
the cause of continental drift, most geologists rejected his theory. For nearly
half a century, from the 1920s to the 1960s, most scientists paid little attention
to the idea of continental drift. Then new evidence about Earth’s structure
led scientists to reconsider Wegener’s bold theory.
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Plate Tectonics
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Name ____________________________
Plate Tectonics
■
Date ___________________
Class ____________
Section Summary
Sea-Floor Spreading
Guide for Reading
■ What is the process of sea-floor spreading?
What is the evidence for sea-floor spreading?
■
What happens at deep-ocean trenches?
The longest chain of mountains in the world is the system of mid-ocean
ridges. In the mid-1900s, scientists mapped the mid-ocean ridges using
sonar. Sonar is a device that bounces sound waves off underwater objects
and then records the echoes of these sound waves. The mid-ocean ridges
curve along the sea floor, extending into all of Earth’s oceans. Most of the
mountains in the mid-ocean ridges lie hidden under hundreds of meters of
water. A steep-sided valley splits the top of some mid-ocean ridges.
Earth’s ocean floors move like conveyor belts, carrying the continents
along with them. This movement begins at a mid-ocean ridge. A ridge forms
along a crack in the oceanic crust. At a mid-ocean ridge, molten material
rises from the mantle and erupts. The molten material then spreads out,
pushing older rock to both sides of the ridge. As the molten material cools,
it forms a strip of solid rock in the center of the ridge. Then more molten
material splits apart the strip of solid rock that formed before, pushing it
aside. This process, called sea-floor spreading, continually adds new
material to the ocean floor.
Scientists have found strange rocks shaped like pillows in the central
valley of mid-ocean ridges. Such rocks can form only if molten material
hardens quickly after erupting under water. The presence of these rocks
supports the theory of sea-floor spreading. More support came when
scientists discovered that the rock that makes up the ocean floor lies in a
pattern of magnetized “stripes.” The pattern is the same on both sides of the
ridge. These stripes hold a record of reversals in Earth’s magnetic field. The
final proof of sea-floor spreading came from rock samples obtained by
drilling into the ocean floor. Scientists found that the farther from a ridge the
rocks were taken, the older they were.
The ocean floor does not just keep spreading. Instead, it sinks beneath
deep underwater canyons called deep-ocean trenches. Where there are
trenches, subduction takes place. Subduction is the process by which the
ocean floor sinks beneath a deep-ocean trench and back into the mantle. At
deep-ocean trenches, subduction allows part of the ocean floor to sink
back into the mantle, over tens of millions of years.
The processes of subduction and sea-floor spreading can change the size
and shape of the oceans. Because of these processes, the ocean floor is
renewed about every 200 million years. The Pacific Ocean is shrinking. Its
many trenches are swallowing more ocean crust than the mid-ocean ridge is
producing. The Atlantic Ocean is expanding. In most places, the oceanic
crust of the Atlantic Ocean is attached to continental crust. As the Atlantic’s
floor spreads, the continents along its edges also move.
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Plate Tectonics
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Name ____________________________
Plate Tectonics
■
Date ___________________
Class ____________
Section Summary
The Theory of Plate Tectonics
Guide for Reading
■ What is the theory of plate tectonics?
■
What are the three types of plate boundaries?
Earth’s lithosphere is broken into separate sections called plates. The plates fit
closely together along cracks in the crust. They carry the continents, or parts
of the ocean floor, or both. Plate tectonics is the geological theory that states
that pieces of Earth’s lithosphere are in constant, slow motion, driven by
convection currents in the mantle. A scientific theory is a well-tested concept
that explains a wide range of observations. The theory of plate tectonics
explains the formation, movement, and subduction of Earth’s plates.
The plates float on top of the asthenosphere. Convection currents rise in
the asthenosphere and spread out beneath the lithosphere, causing the
movement of Earth’s plates. As the plates move, they produce changes in
Earth’s surface, including volcanoes, mountain ranges, and deep-ocean
trenches. The edges of different pieces of the lithosphere meet at lines called
plate boundaries. Faults—breaks in Earth’s crust where rocks have slipped
past each other—form along these boundaries.
There are three types of plate boundaries: transform boundaries,
divergent boundaries, and convergent boundaries. The plates move at
amazingly slow rates, from about 1 to 24 centimeters per year. They have
been moving for tens of millions of years. A transform boundary is a place
where two plates slip past each other, moving in opposite directions.
Earthquakes occur frequently along these boundaries. The place where two
plates move apart, or diverge, is called a divergent boundary. Most
divergent boundaries occur at the mid-ocean ridge. When a divergent
boundary develops on land, two slabs of Earth’s crust slide apart. A deep
valley called a rift valley forms along the divergent boundary. The place
where two plates come together, or converge, is a convergent boundary.
When two plates converge, the result is called a collision. When two plates
collide, the density of the plates determines which one comes out on top.
Oceanic crust is more dense than continental crust.
When two plates carrying oceanic crust meet at a trench, the plate that is
less dense dives under the other plate and returns to the mantle. This is the
process of subduction. When a plate carrying oceanic crust collides with a
plate carrying continental crust, the more dense oceanic plate plunges beneath
the continental plate through the process of subduction. When two plates
carrying continental crust collide, subduction does not take place because
both plates are mostly low-density granite rock. Instead, the plates crash headon. The collision squeezes the crust into mighty mountain ranges.
About 260 million years ago, the continents were joined together in the
supercontinent Pangaea. About 225 million years ago, Pangaea began to
break apart. Since then, the continents have moved to their present locations.
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Name ____________________________
Earthquakes
■
Date ___________________
Class ____________
Section Summary
Forces in Earth’s Crust
Guide for Reading
■
How does stress in the crust change Earth’s surface?
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Where are faults usually found, and why do they form?
■
What land features result from the forces of plate movement?
The movement of Earth’s plates creates enormous forces that squeeze or pull
the rock in the crust. A force that acts on rock to change its shape or volume
is stress. Stress adds energy to the rock. The energy is stored in the rock until
it changes shape or breaks.
Three different kinds of stress can occur in the crust—tension,
compression, and shearing. Tension, compression, and shearing work over
millions of years to change the shape and volume of rock. Tension pulls on
the crust, stretching rock so that it becomes thinner in the middle.
Compression squeezes rock until it folds or breaks. Shearing pushes a mass
of rock in two opposite directions.
When enough stress builds up in rock, the rock breaks, creating a fault.
A fault is a break in the rock of the crust where rock surfaces slip past each
other. Most faults occur along plate boundaries, where the forces of plate
motion push or pull the crust so much that the crust breaks. There are
three main types of faults: normal faults, reverse faults, and strike-slip
faults.
Tension causes a normal fault. In a normal fault, the fault is at an angle,
and one block of rock lies above the fault while the other block lies below the
fault. The block of rock that lies above is called the hanging wall. The rock
that lies below is called the footwall. Compression causes reverse faults. A
reverse fault has the same structure as a normal fault, but the blocks move
in the opposite direction. Shearing creates strike-slip faults. In a strike-slip
fault, the rocks on either side of the fault slip past each sideways, with little
up or down motion.
Over millions of years, the forces of plate movement can change a flat
plain into landforms such as anticlines and synclines, folded mountains,
fault-block mountains, and plateaus. A fold in rock that bends upward into
an arch is an anticline. A fold in rock that bends downward to form a valley
is a syncline. Anticlines and synclines are found on many parts of the Earth’s
surface where compression forces have folded the crust. The collision of two
plates can cause compression and folding of the crust over a wide area.
Where two normal faults cut through a block of rock, fault movements may
push up a fault-block mountain. The forces that raise mountains can also
uplift, or raise plateaus. A plateau is a large area of flat land elevated high
above sea level.
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Name ____________________________ Date ____________________ Class ____________
Mapping Earth’s Surface
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Section Summary
Models of Earth
Guide for Reading
■ How do maps and globes represent Earth’s surface?
■
What reference lines are used to locate points on Earth?
■
What are three common map projections?
A map is a flat model of all or part of Earth’s surface as seen from above.
A globe is a sphere that represents Earth’s entire surface. Maps and globes
are drawn to scale and use symbols to represent topography and other
features on Earth’s surface. A map’s scale relates distance on a map to a
distance on Earth’s surface. Mapmakers use shapes and pictures called
symbols to stand for features on Earth’s surface. A map’s key, or legend, is
a list of all the symbols used on the map with an explanation of their
meaning.
Most maps and globes show a grid. Because Earth is a sphere, the grid
curves to cover the entire planet. Two of the lines that make up the grid, the
equator and prime meridian, are the baselines for measuring distances on
Earth’s surface.
To locate positions on Earth’s surface, scientists use units called degrees.
1
A degree (°) is 360
of the way around a circle. On Earth’s surface, each
degree is a measure of an angle formed by lines drawn from the center of
Earth to points on the surface.
Halfway between the North and South Poles, the equator forms an
imaginary line that circles Earth. The equator divides Earth into the
Northern and Southern Hemispheres. A hemisphere is one half of the
sphere that makes up Earth’s surface. Another imaginary line, called the
prime meridian, makes a half circle from the North Pole to the South Pole
through Greenwich, England. Places east of the prime meridian are in the
Eastern Hemisphere. Places west of the prime meridian are in the Western
Hemisphere.
The lines of latitude and longitude form a grid that can be used to find
locations anywhere on Earth. The equator is the starting line for measuring
latitude, or distance in degrees north or south of the equator. The distance in
degrees east or west of the prime meridian is called longitude.
To show Earth’s curved surface on a flat map, mapmakers use map
projections. A map projection is a framework of lines that helps in
transferring points on Earth’s three-dimensional surface onto a flat map.
Three common map projections are the Mercator projection, the equalarea projection, and the conic projection. In a Mercator projection, all the
lines of latitude and longitude appear as straight, parallel lines that form a
rectangle. Shapes and sizes of landmasses near the poles are distorted. An
equal-area projection shows areas correctly, but distorts some shapes around
the edges of the map. In a conic projection, lines of longitude appear as
straight lines while lines of latitude are curved.
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Name ____________________________ Date ____________________ Class ____________
Mapping Earth’s Surface
■
Section Summary
Maps and Computers
Guide for Reading
■ How does computer mapping differ from earlier ways of mapmaking?
What sources of data are used in making computer maps?
For centuries, mapmakers drew maps by hand. Explorers made maps by
sketching coastlines as seen from their ships. More accurate maps were
made by locating points on Earth’s surface in a process called surveying. In
surveying, mapmakers determine distances and elevations using
instruments and the principles of geometry. During the twentieth century,
people learned to make highly accurate maps using photographs taken from
airplanes.
Since the 1970s, computers have revolutionized mapmaking. With
computers, mapmakers can store, process, and display map data
electronically. All of the data used in computer mapping must be written in
numbers. The process by which mapmakers convert the location of map
points to numbers is called digitizing. The digitized data can be easily
displayed on a computer screen, modified, and printed out in map form.
Computers can automatically make maps that might take a person hours
to draw by hand. Computers produce maps using data from many sources,
including satellites and the Global Positioning System. Much of the data
used in computer mapping is gathered by satellites. Mapping satellites use
electronic devices to collect computer data about the land surface. Pictures
of the surface based on these data are called satellite images.
A satellite image is made up of thousands of tiny dots called pixels. Each
pixel in a satellite image contains information on the color and brightness of
a small part of Earth’s surface. When the satellite image is printed, the
computer translates these digitized data into colors.
Today, mapmakers can collect data for maps using the Global
Positioning System, or GPS. The Global Positioning System is a method of
finding latitude, longitude, and elevation of points on Earth’s surface by
using a network of satellites.
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Mapping
Earth’s Surface
■
Name ____________________________ Date ____________________ Class ____________
Mapping Earth’s Surface
■
Section Summary
Topographic Maps
Guide for Reading
A topographic map is a map showing the surface features of an area.
Topographic maps use symbols to portray the land as if you were looking
down on it from above. Topographic maps provide highly accurate
information on the elevation, relief, and slope of the ground surface.
Mapmakers use contour lines to represent elevation, relief, and slope
on topographic maps. On topographic maps a contour line connects points
of equal elevation. The change in elevation from contour line to contour line
is called the contour interval. The contour interval for a given map is always
the same. Usually, every fifth contour line, known as an index contour, is
darker and heavier than the others. Index contours are labeled with the
elevation above sea level in round units, such as 2,000 feet above sea level.
To read a topographic map, you must familiarize yourself with
the map’s scale and symbols and interpret the map’s contour lines.
Topographic maps usually are large-scale maps. A large-scale map is one
that shows a close-up view of part of Earth’s surface. In the United States,
most topographic maps are at a scale of 1:24,000, or 1 centimeter equals 0.24
kilometers. At this scale, a map can show the details of elevation and features
such as rivers and coastlines. Large buildings, airports, and major highways
appear as outlines at the correct scale. Symbols are used to show houses and
other small features.
On a topographic map, closely spaced contour lines indicate steep
slopes. Widely spaced contour lines indicate gentle slopes. A contour line
that forms a closed loop with no other contour lines inside it indicates a
hilltop. A closed loop with dashes inside indicates a depression. V-shaped
contour lines pointing downhill indicate a ridge line. V-shaped contour lines
pointing uphill indicate a valley.
Topographic maps have many uses in science and engineering,
business, government, and everyday life. Businesses use them, and so do
cities and towns. Topographic maps have recreational uses as well.
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Name ____________________________ Date ____________________ Class ____________
Weathering and Soil Formation
■
Section Summary
Rocks and Weathering
Guide for Reading
■ How do weathering and erosion affect Earth’s surface?
■
What are the causes of mechanical weathering and chemical weathering?
■
What determines how fast weathering occurs?
Weathering is the process that breaks down rock and other substances of
Earth’s surface. Erosion is the removal of rock particles by wind, water, ice,
or gravity. Weathering and erosion work together continuously to wear
down and carry away the rocks at Earth’s surface. The weathering and
erosion that geologists observe today also shaped Earth’s surface millions of
years ago. How do geologists know this? Geologists make inferences based
on the principle of uniformitarianism.This principle states that the same
processes that operate today operated in the past.
There are two kinds of weathering: mechanical weathering and chemical
weathering. Both types of weathering act slowly, but over time they break
down even the biggest, hardest rocks. The type of weathering in which rock
is physically broken into smaller pieces is called mechanical weathering.
The causes of mechanical weathering include freezing and thawing,
release of pressure, plant growth, actions of animals, and abrasion. The
term abrasion refers to the grinding away of rock by rock particles carried
by water, ice, wind, or gravity.
In cool climates, water expands when it freezes and acts as a wedge. This
process is called ice wedging. With repeated freezing and thawing, cracks
slowly expand until pieces of rock break off.
Another type of weathering that attacks rocks is chemical weathering, a
process that breaks down rock through chemical changes. The causes of
chemical weathering include action of water, oxygen, carbon dioxide,
living organisms, and acid rain. Chemical weathering can produce new
minerals as it breaks down rock. Chemical and mechanical weathering often
work together. As mechanical weathering breaks rocks into pieces, more
surface area becomes exposed to chemical weathering.
Water is the most important cause of chemical weathering. Water
weathers rock by dissolving it. The oxygen in air is an important cause of
chemical weathering. Iron combines with oxygen in the presence of water in
a process called oxidation. The product of oxidation is rust.
The most important factors that determine the rate at which
weathering occurs are the type of rock and the climate. Some types of rock
weather more rapidly than others. For example, some rock weathers easily
because it is permeable, which means that it is full of air spaces that allow
water to seep through it.
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Name ____________________________ Date ____________________ Class ____________
Weathering and Soil Formation
■
Section Summary
How Soil Forms
Guide for Reading
■ What is soil made of, and how does soil form?
■
How do scientists classify soils?
■
What is the role of plants and animals in soil formation?
© Pearson Education, Inc., publishing as Pearson Prentice Hall. All rights reserved.
Weathering and
Soil Formation
Soil is the loose, weathered material on Earth’s surface in which plants can
grow. Bedrock is the solid layer of rock beneath the soil.
Soil is a mixture of rock particles, minerals, decayed organic material,
air, and water. The decayed organic material in soil is humus, a dark-colored
substance that forms as plant and animal remains decay. Humus helps create
spaces in soil for air and water that plants must have. The fertility of soil is
a measure of how well the soil supports plant growth.
Soil texture depends on the size of individual particles. The largest soil
particles are gravel. Next in size are sand particles, followed by silt particles.
Clay particles are the smallest. Texture is important for plant growth. Plants
can “drown” for lack of air in clay soil, and they may die from lack of water
in sandy soil. The best soil for growing most plants is loam, which is soil that
is made up of about equal parts of clay, sand, and silt.
Soil forms as rock is broken down by weathering and mixes with other
materials on the surface. It is constantly formed wherever bedrock is
exposed. Soil formation continues over a long period, and gradually soil
develops layers called horizons. A soil horizon is a layer of soil that differs
in color and texture from the layers above or below it. The top layer, the A
horizon, is made up of topsoil, a crumbly, dark brown soil that is a mixture
of humus, clay, and other minerals. The next layer, the B horizon, often called
subsoil, usually consists of clay and other particles washed down from the
A horizon, but little humus. Below that layer is the C horizon, which
contains only partly weathered rock.
Scientists classify different types of soil into major groups based on
climate, plants, and soil composition. The most common plants found in a
region are also used to help classify the soil. Major soil types in North
America include forest, prairie, desert, mountain, tundra, and tropical soils.
Soil teems with living things. Some soil organisms make humus, the
material that makes soil fertile. Other soil organisms mix the soil and
make spaces in it for air and water. Plants contribute most of the organic
remains that form humus. The leaves that plants shed form a loose layer on
the ground called litter. Humus forms in a process called decomposition, in
which organisms that live in the soil turn dead organic material into humus.
The organisms that break the remains of dead organisms into smaller pieces
and digest them with chemicals are called decomposers. Fungi, bacteria,
worms, and other organisms are the main soil decomposers. Earthworms do
most of the work of mixing humus with other materials in soil. Earthworms
and burrowing animals also help aerate, or mix air into, the soil.
Name ____________________________ Date ____________________ Class ____________
Erosion and Deposition
■
Section Summary
Changing Earth’s Surface
Guide for Reading
■ What processes wear down and build up Earth’s surface?
■
What causes the different types of mass movement?
Erosion is the process by which natural forces move weathered rock and soil
from one place to another. Gravity, running water, glaciers, waves, and wind
all cause erosion. The material moved by erosion is sediment. When the
agents of erosion lay down sediment, deposition occurs. Deposition
changes the shape of the land. Weathering, erosion, and deposition act
together in a cycle that wears down and builds up Earth’s surface. Erosion
and deposition are at work everywhere on Earth. Erosion and deposition are
never-ending.
Gravity pulls everything toward the center of Earth. Gravity is the force
that moves rock and other materials downhill. Gravity causes mass
movement, any one of several processes that move sediment downhill. The
different types of mass movement include landslides, mudslides, slump,
and creep. Mass movement can be rapid or slow.
The most destructive type of mass movement is a landslide, which
occurs when rock and soil slide quickly down a steep slope. Some landslides
contain huge masses of rock, while others may contain only a small amount
of rock and soil.
A mudflow is the rapid downhill movement of a mixture of water, rock,
and soil. The amount of water in a mudflow can be as high as 60 percent.
Mudflows often occur after heavy rains in a normally dry area. In clay soils
with a high water content, mudflows may occur even on very gentle slopes.
An earthquake can trigger both mudflows and landslides.
In the type of mass movement known as a slump, a mass of rock and soil
suddenly slips down in one large mass. It looks as if someone pulled the
bottom out from under part of the slope. A slump often occurs when water
soaks the base of a mass of soil that is rich in clay.
Creep is the very slow downhill movement of rock and soil. It occurs
most often on gentle slopes. Creep often results from the freezing and
thawing of water in cracked layers of rock beneath the soil. Creep is so slow
that you can barely notice it, but you can see its effects in objects such as
telephone poles, gravestones, and fenceposts. Creep may tilt these objects at
spooky angles.
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Erosion and Deposition
■
Section Summary
The Force of Moving Water
Guide for Reading
■ What enables water to do work?
■
How does sediment enter rivers and streams?
■
What factors affect a river’s ability to erode and carry sediment?
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Erosion and Deposition
A river’s water has energy. Energy is the ability to do work or cause change.
There are two kinds of energy. Potential energy is energy that is stored and
waiting to be used later. Kinetic energy is the energy an object has due to its
motion. As gravity pulls water down a slope, the water ’s potential energy
changes to kinetic energy that can do work. All along a river, moving water
causes changes. A river is always moving sediment from place to place. At
the same time, a river is also eroding its banks and its valley.
In the process of water erosion, water picks up and moves sediment.
Sediment can enter rivers in a number of ways. Most sediment washes or
falls into a river as a result of mass movement and runoff. Other sediment
erodes from the bottom or sides of the river. Wind may also drop sediment
into the water. Abrasion is another process by which a river obtains sediment.
Abrasion is the wearing away of rock by a grinding action.
The amount of sediment that a river carries is its load. Gravity and the
force of moving water cause sediment to move downstream.
A river’s slope, its volume of flow, and the shape of its streambed all
affect how fast the river flows and how much sediment it can erode. A fastflowing river carries more and larger particles of sediment. When a river
slows down, it deposits some of its sediment load. Generally, as a river’s
slope increases, its speed also increases. A river’s slope is the amount the
river drops toward sea level over a given distance. If a river ’s speed
increases, its sediment load and power to erode may increase. A river’s flow
is the volume of water that moves past a point on the river at any given time.
As more water flows through a river, its speed increases.
Friction is the force that opposes the motion of one surface as it moves across
another surface. Friction affects a river’s speed. Where a river is deep, less water
comes in contact with the streambed. This reduces friction and allows the river
to flow faster. In a shallow river, there is more friction, which reduces the river’s
speed. A streambed is often full of boulders and other obstacles. This roughness
increases friction and reduces a river’s speed. The water moves every which
way in a type of movement called turbulence. Turbulence slows a stream’s flow,
but a turbulent stream has great power to erode.
The shape of a river affects the way it deposits sediment. Deposition
occurs along the sides of a river, where the water moves more slowly. If a
river curves, the water moves fastest on the outside of the curve. There, the
river erodes. On the inside of the curve, where the speed is slowest, the river
deposits sediment.
Name ____________________________ Date ____________________ Class ____________
Erosion and Deposition
■
Section Summary
Glaciers
Guide for Reading
■ What are the two kinds of glaciers?
■
How does a valley glacier form and move?
■
How do glaciers cause erosion and deposition?
A glacier is any large mass of ice that moves slowly over land. There are two
kinds of glaciers—continental glaciers and valley glaciers. A continental
glacier is a glacier that covers much of a continent or large island. Today,
continental glaciers cover about 10 percent of Earth’s land, including
Antarctica and most of Greenland. Continental glaciers can flow in
all directions.
Many times in the past, continental glaciers have covered large parts
of Earth’s surface. These times are known as ice ages. For example, about
2.5 million years ago, continental glaciers covered about a third of Earth’s
land. The glaciers advanced and retreated, or melted back, several times.
They retreated for the last time about 10,000 years ago. A valley glacier is a
long, narrow glacier that forms when snow and ice build up high in a
mountain valley. Valley glaciers are found on high mountains.
Glaciers can form only in an area where more snow falls than melts.
When the depth of snow and ice reaches more than 30 to 40 meters, gravity
begins to pull the glacier downhill. Valley glaciers move, or flow, at a rate
of a few centimeters to a few meters per day. Sometimes a valley glacier
slides down more quickly in what is called a surge.
The movement of a glacier changes the land beneath it. Although glaciers
work slowly, they are a major force of erosion. The two processes by which
glaciers erode the land are plucking and abrasion. As a glacier flows over
the land, it picks up rocks in a process called plucking. Plucking can move
even huge boulders. Many rocks remain on the bottom of a glacier, and the
glacier drags them across the land. This process, called abrasion, gouges and
scratches the bedrock.
A glacier gathers huge amounts of rock and soil as it moves. When a
glacier melts, it deposits the sediment it eroded from the land, creating
various landforms. The mixture of sediments that a glacier deposits directly
on the surface is called till. The till deposited at the edges of a glacier forms
a ridge called a moraine. A terminal moraine is the ridge of till at the farthest
point reached by a glacier. Retreating glaciers also create features called
kettles. A kettle is a small depression that forms when a chunk of ice is left
in glacial till. When the ice melts, the kettle remains. A kettle that is filled
with water is called a kettle lake.
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A Trip Through Geologic Time
■
Section Summary
Fossils
Guide for Reading
■ How do fossils form?
■
What are the different kinds of fossils?
■
What does the fossil record tell about organisms and environments of
the past?
Fossils are the preserved remains or traces of living things. Fossils provide
evidence of how life has changed over time. Most fossils form when living
things die and are buried by sediments. The sediments slowly harden into
rock and preserve the shapes of the organisms. Fossils are usually found in
sedimentary rock, the type of rock that is made of hardened sediment.
Most fossils form from animals or plants that once lived in or near quiet
water such as swamps, lakes, or shallow seas. When an organism dies,
generally only its hard parts leave fossils. Fossils found in rock include
molds and casts, petrified fossils, carbon films, and trace fossils. Other
fossils form when the remains of organisms are preserved in substances
such as tar, amber, or ice.
The most common fossils are molds and casts. A mold is a hollow area in
sediment in the shape of an organism or part of an organism. A mold forms
when the hard part of an organism, such as a shell, is buried in sediment.
Later, water carrying dissolved minerals may seep into the empty space of a
mold. If the water deposits the minerals there, the result is a cast, a solid copy
of the shape of an organism. Petrified fossils are fossils in which minerals
replace all or part of an organism. Another type of fossil is a carbon film, an
extremely thin coating of carbon on rock. Trace fossils provide evidence of
the activities of ancient organisms. Fossil footprints, trails, and burrows are
examples of trace fossils. Some processes preserve the remains of organisms
with little or no change. Organisms can be preserved in tar, amber, or ice.
Scientists who study fossils are called paleontologists. Paleontologists
collect and classify fossils. Together, all the information that paleontologists
have gathered about past life is called the fossil record. The fossil record
provides evidence about the history of life on Earth. The fossil record also
shows that groups of organisms have changed over time. It also reveals
that fossils occur in a particular order, showing that life on Earth has
evolved, or changed. Thus, the fossil record provides evidence to support
the theory of evolution. A scientific theory is a well-tested concept that
explains a wide range of observations. Evolution is the gradual change in
living things over long periods of time. The fossil record shows that millions
of types of organisms have evolved. Some have become extinct. A type of
organism is extinct if it no longer exists and will never again live on Earth.
Fossils provide evidence of Earth’s climate in the past. Paleontologists also
use fossils to learn about past environments and changes in Earth’s surface.
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A Trip Through Geologic Time
■
Section Summary
The Relative Age of Rocks
Guide for Reading
■ What is the law of superposition?
■
How do geologists determine the relative age of rocks?
■
How are index fossils useful to geologists?
The relative age of a rock is its age compared with the ages of other rocks.
The absolute age of a rock is the number of years since the rock formed. The
sediment that forms sedimentary rocks is deposited in flat layers. Over
years, the sediment hardens and changes into sedimentary rock. These rock
layers provide a record of Earth’s geologic history.
It can be difficult to determine the absolute age of a rock. Geologists use
the law of superposition to determine the relative ages of sedimentary rock
layers. According to the law of superposition, in horizontal sedimentary
rock layers the oldest layer is at the bottom. Each higher layer is younger
than the layer below it.
There are other clues to the relative ages of rocks. To determine relative
age, geologists also study extrusions and intrusions of igneous rock,
faults, and gaps in the geologic record. Igneous rock forms when magma or
lava hardens. Lava that hardens on the surface is called an extrusion. The
rock layers below an extrusion are always older than the extrusion. Beneath
the surface, magma may push into bodies of rock. There, the magma cools
and hardens into a mass of igneous rock called an intrusion. An intrusion is
always younger than the rock layers around and beneath it.
More clues come from the study of faults. A fault is a break in Earth’s
crust. A fault is always younger than the rock it cuts through. The surface
where new rock layers meet a much older rock surface beneath them is
called an unconformity. An unconformity is a gap in the geologic record.
An unconformity shows where some rock layers have been lost because
of erosion.
To date rock layers, geologists first give a relative age to a layer of rock
at one location and then give the same age to matching layers at other
locations. Certain fossils, called index fossils, help geologists match rock
layers. To be useful as an index fossil, a fossil must be widely distributed
and represent a type of organism that existed only briefly. Index fossils are
useful because they tell the relative ages of the rock layers in which they
occur. Geologists use particular types of organisms, such as ammonites, as
index fossils. Ammonites were a group of hard-shelled animals that evolved
in shallow seas more than 500 million years ago. They later became extinct.
Ammonite fossils have been found in many different places.
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A Trip Through Geologic Time
■
Section Summary
Early Earth
Guide for Reading
■ When did Earth form?
■
How did Earth’s physical features develop during Precambrian Time?
■
What were early Precambrian organisms like?
Scientists hypothesize that Earth formed at the same time as the other
planets and the sun, roughly 4.6 billion years ago. According to this
hypothesis, Earth collided with a large object. The collision threw a large
amount of material from both bodies into orbit around Earth. This material
combined to form the moon. Scientists think that Earth began as a ball of
dust, rock, and ice in space. Gravity pulled this mass together. As Earth grew
larger, its gravity increased, pulling in nearby dust, rock, and ice. As the
growing Earth traveled around the sun, its gravity also captured gases such
as hydrogen and helium. However, this first atmosphere was lost when the
sun released a strong burst of particles.
During the first several hundred million years of Precambrian Time,
an atmosphere, oceans, and continents began to form. After Earth lost its
first atmosphere, a second atmosphere formed. The new atmosphere was
made up mostly of carbon dioxide, nitrogen, and water vapor. Volcanic
eruptions released carbon dioxide, water vapor, and other gases from
Earth’s interior. Collisions with comets added other gases to the atmosphere.
A comet is a ball of dust and ice that orbits the sun.
At first, Earth’s surface was too hot for water to remain as a liquid. All
water evaporated into water vapor. However, as Earth’s surface cooled, the
water vapor began to condense to form rain. Gradually, rainwater began to
accumulate to form an ocean. Over time, the oceans affected the composition
of the atmosphere by absorbing much of the carbon dioxide.
Within 500 million years of Earth’s formation, continents formed.
Scientists have found that the continents move very slowly over Earth’s
surface because of forces inside Earth. This process is called continental
drift. The movement is slow—only a few centimeters per year. Over billions
of years, Earth’s landmasses have repeatedly formed, broken apart, and then
crashed together again, forming new continents.
Scientists cannot pinpoint when or where life began on Earth. But
scientists have found fossils of single-celled organisms in rocks that
formed about 3.5 billion years ago. These earliest life forms were probably
similar to present-day bacteria. Scientists hypothesize that all other forms
of life on Earth arose from these simple organisms.
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Earth, Moon, and Sun
■
Section Summary
Earth in Space
Guide for Reading
■ How does Earth move in space?
■
What causes the cycle of seasons on Earth?
The study of the moon, stars, and other objects in space is called astronomy.
Ancient astronomers studied the movements of the sun and moon. They
thought Earth was standing still and the sun and moon were moving. The
sun and moon seem to move mainly because Earth is rotating on its axis, the
imaginary line that passes through Earth’s center and the North and South
poles. The spinning of Earth on its axis is called its rotation. Earth’s rotation
on its axis causes day and night. It takes Earth about 24 hours to rotate once
on its axis.
The movement of one object around another object is called revolution.
Earth completes one revolution around the sun once every year. Earth’s path
as it revolves around the sun is called its orbit. Earth’s orbit is a slightly
elongated circle, or ellipse.
Many cultures have tried to make a workable calendar. A calendar is a
system of organizing time that defines the beginning, length, and divisions
of a year. This is not easy because Earth takes about 365 1/4 days to circle the
sun, and 12 moon cycles make up fewer days than a calendar year.
Sunlight hits Earth’s surface most directly at the equator. Closer to the
poles, sunlight hits Earth’s surface at an angle. That is why it is generally
warmer near the equator than near the poles.
Earth has seasons because its axis is tilted as it moves around the sun.
Earth’s axis is tilted at an angle of 23.5° from vertical. As Earth revolves
around the sun, its axis is tilted away from the sun for part of the year and
toward the sun for part of the year. When the north end of Earth’s axis is
tilted toward the sun, the Northern Hemisphere has summer. At the same
time, the south end of Earth’s axis is tilted away from the sun. As a result, the
Southern Hemisphere has winter. The hemisphere tilted toward the sun has
more daylight hours than the hemisphere tilted away from the sun. The
combination of direct rays and more hours of sunlight heats the surface more
than at any other time of the year.
On two days each year, the sun reaches its greatest distance north or
south of the equator. Each of these days is known as a solstice. Halfway
between the solstices, neither hemisphere is tilted toward the sun. On those
two days, the noon sun is directly overhead at the equator. Each of these
days is known as an equinox, meaning “equal night.” During an equinox,
the length of nighttime and daytime are about the same.
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Earth, Moon, and Sun
■
Section Summary
Gravity and Motion
Guide for Reading
■ What determines the strength of the force of gravity between two objects?
■
What two factors combine to keep the moon and Earth in orbit?
Earth revolves around the sun in a nearly circular orbit. The moon orbits
Earth in the same way. But what keeps Earth and the moon in orbit? Why
don’t they just fly off into space? The first person to answer these questions
was the English scientist Isaac Newton. Newton told a story about how
watching an apple fall from a tree in 1666 had made him think about the
moon’s orbit. Newton realized that there must be a force acting between
Earth and the moon that kept the moon in orbit. A force is a push or a pull.
Newton hypothesized that the force that pulls an apple to the ground
also pulls the moon toward Earth, keeping it in orbit. This force, called
gravity, attracts all objects toward each other. In Newton’s day, most
scientists thought that forces on Earth were different from those elsewhere
in the universe. Although Newton did not discover gravity, he was the first
to realize that gravity occurs everywhere. Newton’s law of universal
gravitation states that every object in the universe attracts every other object.
The strength of gravity is measured in units called newtons, named after
Isaac Newton. The strength of the force of gravity between two objects
depends on two factors: the masses of the objects and the distance
between them. According to the law of universal gravitation, all of the
objects around you are pulling on you. Why don’t you notice this pull?
Because the strength of gravity depends, in part, on the masses of the objects.
Mass is the amount of matter in an object.
Because Earth is so massive, it exerts a much greater force on you than a
book does. Similarly, Earth exerts a gravitational pull on the moon, large
enough to keep the moon in orbit. The force of gravity on an object is known
as its weight. An object’s weight can change depending on its location. For
example, on the moon, you would weigh about one-sixth of your weight on
Earth. This is because the moon is much less massive than Earth, so the pull
of its gravity on you would be much less.
The tendency of an object to resist a change in motion is inertia. Isaac
Newton stated his ideas about inertia as a scientific law. Newton’s first law of
motion says that an object at rest will stay at rest and an object in motion will
stay in motion with a constant speed and direction unless acted on by a force.
Why do Earth and the moon remain in their orbits? Newton concluded
that two factors—inertia and gravity—combine to keep Earth in orbit
around the sun and the moon in orbit around Earth. Earth’s gravity keeps
pulling the moon toward it, preventing the moon from moving in a straight
line. At the same time, the moon keeps moving ahead because of its inertia. If
not for Earth’s gravity, inertia would cause the moon to move off through
space in a straight line. In the same way, Earth revolves around the sun because
the sun’s gravity pulls on it while Earth’s inertia keeps it moving ahead.
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Earth, Moon, and Sun
■
Section Summary
Phases, Eclipses, and Tides
Guide for Reading
■ What causes the phases of the moon?
■
What are solar and lunar eclipses?
■
What causes the tides?
As the moon moves, the positions of the moon, Earth, and the sun change in
relation to each other. The changing relative positions of the moon, Earth,
and the sun cause the phases of the moon, eclipses, and tides.
The moon revolves around Earth about once every 27.3 days. It also
rotates on its own axis about once every 27.3 days. The same side of the
moon always faces Earth. The different shapes of the moon you see from
Earth are called phases. The phase of the moon you see depends on how
much of the sunlit side of the moon faces Earth.
When the moon’s shadow hits Earth or Earth’s shadow hits the moon,
an eclipse occurs. An eclipse occurs when an object in space comes between
the sun and a third object, and casts a shadow on that object. There are two
types of eclipses: solar and lunar.
A solar eclipse occurs when the moon passes between Earth and the sun,
blocking the sunlight from reaching Earth. The moon’s shadow then hits
Earth. So a solar eclipse occurs when a new moon blocks your view of the sun.
The darkest part of the moon’s shadow is called the umbra. From any part of
the umbra, the moon completely blocks light from the sun. Only people in the
umbra see a total solar eclipse. Another part of the shadow is less dark and
larger than the umbra. It is called the penumbra. From within the penumbra,
people see a partial eclipse because part of the sun is still visible.
A lunar eclipse occurs at a full moon when Earth is directly between the
moon and the sun. During a lunar eclipse, Earth’s shadow falls on the
moon. Earth’s shadow also has an umbra and a penumbra. When the moon
is completely within Earth’s umbra, you see a total lunar eclipse. A partial
lunar eclipse happens when the moon moves partly into Earth’s umbra.
Tides are the rise and fall of the ocean’s water every 12.5 hours or so. The
force of gravity pulls the moon and Earth toward each other. Tides are
caused mainly by differences in how much the moon pulls on different
parts of Earth. As Earth rotates, the moon’s gravity pulls water toward the
point on Earth’s surface closest to the moon. The moon pulls least on the side
of Earth farthest away. Two tides occur each day because of this difference in
the pull of the moon’s gravity.
Twice a month, the moon, Earth, and the sun are in a straight line. The
combined forces of the gravity of the sun and moon produce an especially
high tide—called a spring tide—and an especially low tide. Also twice a
month, the pull of gravity of the sun and moon are at right angles to each
other. At those times the high tide is lower than usual, and is called a neap
tide. The low tides then are higher than usual.
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The Solar System
■
Section Summary
Observing the Solar System
Guide for Reading
■ What are the geocentric and heliocentric systems?
■
How did Copernicus, Galileo, and Kepler contribute to our knowledge of
the solar system?
■
What objects make up the solar system?
Observers in ancient Greece noticed that although the stars seemed to move,
they stayed in the same position relative to one another. These patterns of
stars, called constellations, kept the same shapes from night to night and
from year to year.
The Greeks thought that Earth was inside a rotating dome called a
celestial sphere. Since the word geo is the Greek word for Earth, an Earthcentered explanation is known as a geocentric system. In a geocentric
system, Earth is at the center of the revolving planets and stars. About
A.D. 140, the Greek astronomer Ptolemy further developed the geocentric
model. Like the earlier Greeks, Ptolemy thought Earth was at the center of a
system of planets and stars. In Ptolemy’s model, however, the planets
moved on small circles that moved on bigger circles. Copernicus was able to
work out the arrangement of the known planets and how they move around
the sun.
A Greek scientist developed the heliocentric system. In a heliocentric
system, Earth and the other planets revolve around the sun.
In the early 1500s, the Polish astronomer Nicolas Copernicus developed
a new model for the motions of the planets. His sun-centered system is also
called heliocentric. Helios is Greek for “sun.” Copernicus was about to work
out the arrangement of the known planets and how they move around the
sun. Later, Galileo used the newly invented telescope to make discoveries
that supported the heliocentric model.
Copernicus thought that the planets’ orbits were circles. He based his
conclusions on observations made by the ancient Greeks. In the late 1500s,
Tycho Brahe made more accurate observations of the planets’ orbits.
Johannes Kepler analyzed Brahe’s data. Kepler found that the orbit of each
planet is an ellipse. An ellipse is an oval shape, which may be elongated or
nearly circular. Kepler used the new scientific evidence gathered by Brahe to
disprove the long-held belief that the planets moved in perfect circles.
Since Galileo’s time, our knowledge of the solar system has increased
dramatically. Today we know that the solar system consists of the sun, nine
planets and their moons, and several kinds of smaller objects that revolve
around the sun.
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The Solar System
■
Section Summary
The Sun
Guide for Reading
■ What are the three layers of the sun’s interior?
■
What are the three layers of the sun’s atmosphere?
■
What features form on or above the sun’s surface?
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The Solar System
The sun’s mass is 99.8 percent of all the mass in the solar system. Because the
sun is so large, its gravity is strong enough to hold all of the planets and
other distant objects in orbit.
Unlike Earth, the sun does not have a solid surface. Like Earth, the sun
has an interior and an atmosphere. The sun’s interior consists of the core,
radiation zone, and convection zone. Each layer has different properties.
The sun produces an enormous amount of energy in its core, or central
region. The sun’s energy comes from nuclear fusion. In the process of
nuclear fusion, hydrogen atoms in the sun join to form helium.
The light and heat produced by the sun’s core first pass through the
middle layer of the sun’s interior, the radiation zone. The radiation zone is
a region of very tightly packed gas where energy is transferred mainly in the
form of electromagnetic radiation.
The convection zone is the outermost layer of the sun’s interior. Hot
gases rise from the bottom of the convection zone and gradually cool as they
approach the top. Cooler gases sink, forming loops of gas that move heat
toward the sun’s surface.
The sun’s atmosphere consists of the photosphere, the chromosphere,
and the corona. The inner layer of the sun’s atmosphere is called the
photosphere. Photo means “light,” so the photosphere is the sphere that
gives off visible light.
At the beginning and end of a solar eclipse, you can see a reddish glow
around the photosphere. This glow comes from the middle layer of the sun’s
atmosphere, the chromosphere. Chromo means “color,” so the chromosphere
is the “color sphere.”
During a total solar eclipse, a fainter layer called the corona is visible.
The corona sends out a stream of electrically charged particles called solar
wind.
Features on or above the sun’s surface include sunspots, prominences,
and solar flares. Sunspots are areas of gas on the sun that are cooler than the
gas around them. Sunspots usually occur in groups. Reddish loops of gas
called prominences link different parts of sunspot regions. Sometimes the
loops in sunspot regions suddenly connect, releasing large amounts of
energy. The energy heats gas on the sun to millions of degrees Celsius,
causing the gas to explode into space. These explosions are known as solar
flares. Solar flares can greatly increase the solar wind.
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The Solar System
■
Section Summary
The Inner Planets
Guide for Reading
■ What characteristics do the inner planets have in common?
■
What are the main characteristics that distinguish each of the
inner planets?
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The Solar System
Mercury, Venus, Earth, and Mars are more similar to one other than they are
to the five outer planets. The four inner planets are small and dense and
have rocky surfaces. These planets are often called the terrestrial planets,
from the Latin word terra, or “earth.”
Earth is unique in our solar system in having liquid water at its
surface. Earth has a suitable atmosphere and temperature range for water to
exist as liquid, gas, or solid. Earth has an atmosphere that is rich in oxygen.
Nearly all of the remaining atmosphere consists of nitrogen, along with
small amounts of other gases such as argon and carbon dioxide. The
atmosphere also includes water vapor.
Mercury is the smallest terrestrial planet and the planet closest to the
sun. Mercury is smaller than Earth’s moon and has no moons of its own. The
planet’s interior is probably made of iron, and its surface has many plains
and craters. Because the planet is so close to the sun, the side facing the sun
reaches temperatures of 430°C. However, the temperature drops to –170°C
at night.
Venus is similar in size and mass to Earth. Venus’ density and internal
structure are similar to Earth’s. But in other ways, Venus and Earth are very
different. Venus rotates from east to west, the opposite direction from most
other planets and moons. The pressure of Venus’s atmosphere is 90 times
greater than the pressure of Earth’s atmosphere. The atmosphere is mostly
carbon dioxide, with clouds partly made up of sulfuric acid. The carbon
dioxide in the planet’s atmosphere traps the sun’s heat, causing the surface
temperature of Venus to be about 460°C. This trapping of heat by the
atmosphere is called the greenhouse effect. Venus is covered with rock,
similar to many rocky areas on Earth. Venus also has many volcanoes and
broad plains formed by lava flows.
Mars is called the “red planet.” Its surface is covered with red dust. The
planet Mars has a very thin atmosphere that is mostly carbon dioxide.
Temperatures on the surface range from –140ºC to 20ºC. Images of Mars
show a variety of features that look as if they were made by ancient streams,
lakes, or floods. Scientists think that a large amount of liquid water flowed
on Mars’s surface in the distant past. At present, liquid water cannot exist
for long on Mars’s surface. However, some water is frozen in the planet’s
two polar ice caps. A large amount of water may be frozen underground.
Like Earth, Mars is tilted on its axis, so its seasons change. Some regions of
Mars have giant volcanoes. Mars has two very small moons, Phobos and
Deimos.
Name ____________________________ Date ____________________ Class ____________
The Solar System
■
Section Summary
The Outer Planets
Guide for Reading
■ What characteristics do the gas giants have in common?
■
What are some characteristics that distinguish each of the outer planets?
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The Solar System
The first four outer planets—Jupiter, Saturn, Uranus, and Neptune—are
much larger and more massive than Earth, and they do not have solid
surfaces. Because these four planets are all so large, they are often called the
gas giants. The fifth outer planet, Pluto, is small and rocky, like the terrestrial
planets.
Like the sun, the gas giants are composed mainly of hydrogen and
helium. Because they are so massive, they exert a much stronger
gravitational force than the terrestrial planets. This prevents their gases from
escaping, so they have thick atmospheres. All of the giants have many
moons and are surrounded by a set of rings. A ring is a thin disk of small
particles of ice and rock.
Jupiter is the largest and most massive planet. Jupiter has a thick
atmosphere made up mainly of hydrogen and helium. An interesting feature
of Jupiter’s atmosphere is its Great Red Spot, a storm that is larger than
Earth. Jupiter probably has a dense core of rock and iron at its center,
surrounded by a thick mantle of liquid hydrogen and helium. Galileo
discovered Jupiter’s four largest moons: Io, Europa, Ganymede, and
Callisto.
Saturn is the second-largest planet in the solar system. Its average
density is less than that of water. The rings around Saturn are made of
chunks of ice and rock. Saturn has the most spectacular rings of any planet.
Uranus is about four times the diameter of Earth and is twice as far from
the sun as Saturn. Uranus looks blue-green because of traces of methane in
its atmosphere. Uranus’s axis of rotation is tilted at an angle of about 90
degrees from the vertical. It rotates from top to bottom instead of from side
to side.
Neptune is a cold, blue planet. Its atmosphere contains visible clouds.
Neptune was discovered as a result of a mathematical prediction.
Astronomers have discovered at least 13 moons orbiting Neptune.
Pluto has a solid surface and is much smaller and denser than the other
outer planets. Pluto has a single moon, Charon. Because Charon is more
than half the size of Pluto, some astronomers consider them to be a double
planet instead of a planet and a moon. Pluto revolves around the sun only
once every 248 Earth years.
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Stars, Galaxies, and the Universe
■
Section Summary
Characteristics of Stars
Guide for Reading
■ How are stars classified?
■
How do astronomers measure distances to the stars?
■
What is an H-R diagram and how do astronomers use it?
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Stars, Galaxies,
and the Universe
When ancient observers around the world looked up at the night sky, they
imagined that groups of stars formed pictures of people or animals. Today,
we call these imaginary patterns of stars constellations.
Astronomers classify stars according to their physical characteristics.
Characteristics used to classify stars include color, temperature, size,
composition, and brightness. Stars vary in their chemical composition.
Astronomers use spectrographs to determine the elements found in stars. A
spectrograph is a device that breaks light into colors and produces an image
of the resulting spectrum.
The brightness of a star depends upon both its size and its
temperature. How bright a star looks from Earth depends on both its
distance from Earth and how bright the star actually is. The brightness of a
star can be described in two different ways: apparent brightness and
absolute brightness. A star’s apparent brightness is its brightness as seen
from Earth. Astronomers can measure apparent brightness fairly easily
using electronic devices. A star’s absolute brightness is the brightness the
star would have if it were at a standard distance from Earth.
Distances on Earth’s surface are often measured in kilometers. However,
distances to the stars are so large that kilometers are not very practical units.
Astronomers use a unit called the light-year to measure distances between
the stars. A light-year is the distance that light travels in one year, about
9.5 million million kilometers.
Standing on Earth looking up at the sky, it may seem as if there is no way
to tell how far away the stars are. However, astronomers have found ways
to measure those distances. Astronomers often use parallax to measure
distances to nearby stars. Parallax is the apparent change in position of an
object when you look at it from different places.
Two important characteristics of stars are temperature and absolute
brightness. Ejnar Hertzsprung and Henry Norris-Russell made a graph to
find out whether these characteristics are related. The graph they made is
called the Hertzsprung-Russell diagram, or H-R diagram. Astronomers use
the H-R diagram to classify stars and to understand how stars change over
time. Most of the stars in the H-R diagram form a diagonal line called the
main sequence. More than 90 percent of all stars, including the sun, are
main-sequence stars. In the main sequence, surface temperature increases as
brightness increases.
Name ____________________________ Date ____________________ Class ____________
Stars, Galaxies, and the Universe
■
Section Summary
Star Systems and Galaxies
Guide for Reading
■ What is a star system?
■
What are the major types of galaxies?
■
How do astronomers describe the scale of the universe?
Our solar system has only one star, the sun. Most stars are members of
groups of two or more stars, called star systems. Star systems that have two
stars are called double stars or binary stars. A system in which one star
periodically blocks the light from another is called an eclipsing binary.
Astronomers have discovered more than 100 planets around other stars.
Most of these new planets are very large. Some scientists think it is possible
that life could be on planets in other solar systems. A few astronomers are
using radio telescopes to search for signals that could not have come from
natural sources.
Many stars belong to larger groups called star clusters. Open clusters
have a loose, disorganized appearance and contain no more than a few
thousand stars. Globular clusters are large groups of older stars. Some may
contain more than a million stars.
A galaxy is a huge group of single stars, star systems, star clusters, dust,
and gas bound together by gravity. Astronomers classify most galaxies into
the following types: spiral, elliptical, and irregular. Galaxies that appear to
have a bulge in the middle and arms that spiral outward, like pinwheels, are
called spiral galaxies. Elliptical galaxies look like round or flattened balls.
Galaxies that do not have regular shapes are known as irregular galaxies.
Quasars are active young galaxies with giant black holes at their centers.
Our solar system is located in a spiral galaxy called the Milky Way. The
Milky Way is usually thought of as a standard spiral galaxy. When you see
the Milky Way at night during the summer, you are looking toward the
center of our galaxy.
Astronomers define the universe as all of space and everything in it.
Since the numbers astronomers use are often very large or very small, they
frequently use scientific notation to describe sizes and distances in the
universe. Scientific notation uses powers of ten to write very large or very
small numbers in shorter form.
The structures in the universe vary greatly in scale. Beyond the solar
system, the sizes of observable objects become much larger. Beyond our
galaxy are billions of other galaxies, many which contain billions of stars.
The Milky Way is a part of a cluster of 50 or so galaxies called the Local
Group. The Local Group is part of the Virgo Supercluster, which contains
hundreds of galaxies.
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Stars, Galaxies, and the Universe
■
Section Summary
The Expanding Universe
Guide for Reading
■ What is the big bang theory?
■
How did the solar system form?
■
What do astronomers predict about the future of the universe?
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Stars, Galaxies,
and the Universe
Astronomers theorize that billions of years ago, the universe was no larger
than the period at the end of this sentence. This tiny universe was incredibly
hot and dense. The universe then exploded in what astronomers call the big
bang. According to the big bang theory, the universe formed in an instant,
billions of years ago, in an enormous explosion.
Edwin Hubble discovered that most of the galaxies are moving away
from us and away from each other. Hubble also discovered that there is a
relationship between the distance to a galaxy and its speed. Hubble’s law
states that the farther away a galaxy is, the faster it is moving away from us.
Hubble’s law provides strong support for the big bang theory.
In 1965, two physicists accidentally detected faint radiation on their
radio telescope. This mysterious glow was coming from all directions in
space. Scientists later concluded that this glow, now known as cosmic
background radiation, is radiation left over from the big bang. Astronomers
estimate that the universe is about 13.7 billion years old.
After the big bang, there was only cold, dark gas and dust where the
solar system is now. About five billion years ago, a giant cloud of gas and
dust collapsed to form our solar system. A large cloud of gas and dust such
as the one that formed our solar system is called a solar nebula. Slowly,
gravity began to pull the solar nebula together. As the solar nebula shrank,
it spun faster and faster and eventually flatted into a rotating disk. Gravity
pulled most of the gas into the center of the disk, where the gas eventually
became hot and dense enough for nuclear fusion to begin. The sun was born.
Meanwhile, in the outer parts of the disk, gas and dust formed small
asteroid-like bodies called planetesimals. These formed the building blocks
of the planets. Planetesimals collided and grew larger by sticking together
and eventually combining to form the planets.
New observations have led many astronomers to conclude that the
universe will likely expand forever. Astronomers have discovered that the
matter that astronomers can see, such as stars and nebulas, makes up as little
as ten percent of the mass of galaxies. The remaining mass in galaxies exists
in the form of dark matter. Dark matter is matter that does not give off
electromagnetic radiation. Astronomers have observed that the expansion of
the universe appears to be accelerating. They infer that a mysterious new
force, which they call dark energy, is causing the expansion of the universe
to accelerate. Most of the universe is thought to be made of dark matter and
dark energy.
Name ____________________________ Date ____________________ Class ____________
Introduction to Matter
■
Section Summary
Changes in Matter
Guide for Reading
■ What is a physical change?
■
What is a chemical change?
■
How are changes in matter related to changes in energy?
Chemistry is the study of changes in matter. Matter can change in two ways.
In a physical change, matter changes its appearance but does not change
into a different substance. A substance that undergoes a physical change is
still the same substance after the change. One example of a physical change
is a change in state. Changing from a solid to a liquid or from a liquid to a
gas is a change in state. Other kinds of physical changes are dissolving,
bending, crushing, and filtering.
The other way that matter can change is a chemical change. In a chemical
change, matter changes into one or more new substances. Unlike a physical
change, a chemical change produces new substances with different
properties from those of the original substances. Combustion, or burning,
is one chemical change. When natural gas burns, it combines with oxygen in
the air to produce carbon dioxide gas and water. Other examples of chemical
change are electrolysis, oxidation, and tarnishing.
Although it may seem like matter disappears when it burns, that is not
what is really happening. It has long been proven that mass is not lost or
gained when matter changes. The law of conservation of mass states that
matter is not created or destroyed in any chemical or physical reaction.
Any time that matter changes, energy is involved. Energy is the ability to
do work or cause change.Every chemical or physical change in matter
includes a change in energy.When ice melts, it absorbs energy from the
surrounding matter.
One kind of energy is thermal energy. Thermal energy is the total energy
of all the particles in an object. Thermal energy always moves from warm
matter to cool matter. Thermal energy is different from temperature.
Temperature does depend on the amount of thermal energy an object has.
Temperature is a measure of the average energy of motion of the particles in
an object.
Thermal energy is the most common form of energy released or absorbed
when matter changes. When ice absorbs thermal energy from its
surroundings, it melts. The melting of ice is an endothermic change. An
endothermic change is a change in which energy is taken in, or absorbed.
When wood burns, energy is given off in the form of heat and light. An
exothermic change releases, or gives off, energy.
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Solids, Liquids, and Gases
■
Section Summary
States of Matter
Guide for Reading
■ What are the characteristics of a solid?
■
What are the characteristics of a liquid?
■
What are the characteristics of a gas?
Matter can be classified as solids, liquids, or gases. These three states of
matter are defined mainly by the way they hold their volume and shape.
A solid has a definite volume and a definite shape. The particles that
make up a solid are packed very closely together. Each particle is tightly
fixed in one position. This fixed, closely packed arrangement of particles
causes a solid to have a definite shape and volume. The particles in a solid
are not completely motionless. The particles vibrate, or move back and forth
slightly.
In many solids, the particles form a regular, repeating pattern. These
patterns create crystals. Solids that are made up of crystals are called
crystalline solids. Salt, sugar, and snow are examples of crystalline solids.
When a crystalline solid is heated, it melts at a specific temperature.
In other solids, called amorphous solids, the particles are not arranged
in a regular pattern. Amorphous solids include plastics, rubber, and glass.
Unlike a crystalline solid, an amorphous solid does not melt at a distinct
temperature. Instead, when it is heated it may become softer and softer or
change into other substances.
A liquid has a definite volume but no shape of its own. A liquid takes on
the shape of its container. Without a container, a liquid spreads into a wide,
shallow puddle. The particles in a liquid are packed almost as closely as in a
solid. However, the particles in a liquid move around one another freely.
Because its particles are free to move, a liquid has no definite shape.
However, it does have a definite volume.
A liquid can flow from place to place. For this reason, a liquid is also
called a fluid, meaning “a substance that flows.”
One property of liquids, surface tension, is caused by the inward pull of
the molecules making up a liquid. This pull brings the molecules on the
surface closer together. This explains why water forms droplets and
supports the weight of certain insects on its surface.
Another property of water, viscosity, is a liquid’s resistance to flowing.
Viscosity depends on the size and shape of the particles of a liquid, and the
attractions between particles. Liquids with high viscosity flow slowly.
Liquids with low viscosity flow quickly.
Unlike solids and liquids, a gas can change volume very easily. The
particles of a gas move at high speeds in all directions. As they move gas
particles spread apart, filling all the space available. Thus, a gas has
neither definite shape nor definite volume.
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Atoms and Bonding
■
Section Summary
Elements and Atoms
Guide for Reading
■ Why are elements sometimes called the building blocks of matter?
■
How did atomic theory develop and change?
Elements are the simplest pure substances. They cannot be broken down
into any other substances. Iron and oxygen are elements. Elements are often
called the building blocks of matter because all matter is composed of one
element or a combination of two or more elements.
Elements usually exist with other elements in the form of compounds. A
compound is a pure substance made of two or more elements that are
combined chemically in a specific ratio. Table salt is an example of a
compound. Elements can also mix with other elements without combining
chemically. A mixture is two or more substances that are in the same place
but are not chemically combined. Air is an example of a mixture. The
smallest particle of an element is an atom.
Scientific theories about the atom began to develop in the 1600s. A
scientific theory is a well-tested idea that explains and connects a wide
range of observations. Theories often include models—physical or other
representations of an idea to help people understand what they cannot
observe directly. Atomic theory grew as a series of models that developed
from experimental evidence. As more evidence was collected, the theory
and models were revised.
John Dalton proposed one of the first models of the atom. Dalton thought
that atoms were like smooth, hard balls that could not be broken into smaller
pieces. In 1897, J. J. Thomson discovered that atoms contained negatively
charged particles. He proposed a model of the atom in which negatively
charged particles were scattered throughout a ball of positive charge. The
negatively charged particles later became known as electrons.
In 1911, Ernest Rutherford did experiments that showed that an atom is
mostly empty space, with electrons moving around a small, positively
charged center. Rutherford called this small positive region in the center of
the atom the nucleus. He determined that the nucleus contained positively
charged particles, which he named protons. In 1913, Niels Bohr revised the
atomic model again. Bohr showed that electrons move around the nucleus
in certain orbits according to their energy. According to Bohr, the electrons
were like planets orbiting the sun. In the 1920s, the atomic model changed
again. Scientists determined that electrons could be anywhere in a cloudlike
region around the nucleus. A region where electrons of the same energy are
likely to be found is called an energy level. In 1932, James Chadwick
discovered another particle in the nucleus of the atom. It was called a
neutron because it is electrically neutral. Since the 1930s, the model of the
atom has not changed much. Scientists conclude the atom consists of a small,
positively charged nucleus, containing protons and neutrons, which is
surrounded by a cloudlike region of negatively charged electrons.
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Chemical Reactions
■
Section Summary
Observing Chemical Change
Guide for Reading
■ How can matter and changes in matter be described?
■
How can you tell when a chemical reaction occurs?
Matter is anything that has mass and takes up space. The study of matter and how
matter changes is called chemistry. Matter can be described in terms of two kinds
of properties—physical properties and chemical properties. Changes in matter
can be described in terms of physical changes and chemical changes.
A physical property is a characteristic of a substance that can be
observed without changing the substance into another substance. The
temperature at which a solid melts is a phyical property. Color, hardness,
and texture are other physical properties.
A chemical property property is a characteristic of a substance that
describes its ability to change into other substances. To observe the chemical
properties of a substance, you must change it into another substance. For
example, to observe the chemical reactivity of magnesium, you can let
magnesium combine with oxygen to form a new substance called
magnesium oxide.
A physical change is any change that alters the form or appearance of a
substance but that does not make the substance into another substance.
Examples of physical changes are bending and cutting.
A change in matter that produces one or more new substances is a
chemical change, or chemical reaction. The burning of gasoline in a car’s
engine is a chemical change. Chemical changes occur when bonds form
between atoms, or when bonds break and new bonds form. As a result,
new substances are produced.
One way to detect chemical reactions is to observe changes in the
properties of the materials involved. Chemical reactions involve two main
kinds of changes you can observe—formation of new substances and
changes in energy. Changes in properties result when new substances form.
A change in color may signal that a new substance has formed. Another
indicator might be the formation of a solid when two solutions are mixed. A
solid that forms from solution during a chemical reaction is called a
precipitate. A third indicator is the formation of a gas when solids or liquids
react. These and other kinds of observable changes in properties may
indicate that a chemical reaction has occurred.
As matter changes in a chemical reaction, it can either absorb or release
energy. One indication that energy has been absorbed or released is a change
in temperature. An endothermic reaction is a reaction in which energy is
absorbed. A reaction that releases energy in the form of heat is called an
exothermic reaction.
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Chemical Reactions
■
Section Summary
Describing Chemical Reactions
Guide for Reading
■ What information does a chemical equation contain?
■
What does the principle of conservation of mass state?
■
What must a balanced chemical equation show?
■
What are three categories of chemical reactions?
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Chemical Reactions
A chemical equation is a short, easy way to show a chemical reaction.
Chemical equations use chemical formulas and other symbols instead of
words to summarize a reaction. All chemical equations have a common
structure. A chemical equation tells you the substances you start with in a
reaction and the substances you get at the end. The substances you have at
the beginning are called the reactants. When the reaction is complete, you
have new substances called the products. The formulas for the reactants are
written on the left side of the equation, followed by an arrow (→). You read
the arrow as “yields.” The formulas for the products are written on the right
side of the equation. When there are two or more reactants or products, they
are separated by plus signs.
The principle called conservation of mass was first demonstrated in the
late 1700s. The principle of conservation of mass states that in a chemical
reaction, the total mass of the reactants must equal the total mass of the
products. In an open system, matter can enter from or escape to the
surroundings. A match burning in the air is an example of an open system.
You cannot measure the mass of all the reactants and products in an open
system. A closed system is a system in which matter cannot enter from or
escape to the surroundings. A sealed plastic bag is an example of a closed
system. A closed system allows you to measure the mass of all reactants and
products in a reaction.
To describe a reaction accurately, a chemical equation must show the
same number of each type of atom on both sides of the equation. An
equation is balanced when it accurately represents conservation of mass. To
balance a chemical equation, you may have to use coefficients. A coefficient
is a number placed in front of a chemical formula in an equation. It tells you
how many atoms or molecules of a reactant or a product take part in the
reaction.
Many chemical reactions can be classified in one of three categories:
synthesis, decomposition, or replacement. When two or more elements or
compounds combine to make a more complex substance, the reaction is
called a synthesis reaction. The reaction of hydrogen and oxygen to make
water is a synthesis reaction. A reaction called a decomposition reaction
breaks down compounds into simpler products. For example, hydrogen
peroxide decomposes into water and oxygen gas. When one element
replaces another in a compound, or when two elements in different
compounds trade places, the reaction is called a replacement reaction.
Name ____________________________ Date ____________________ Class ____________
Chemical Reactions
■
Section Summary
Controlling Chemical Reactions
Guide for Reading
■ How is activation energy related to chemical reactions?
■
What factors affect the rate of a chemical reaction?
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Chemical Reactions
Activation energy is the minimum amount of energy needed to start a
chemical reaction. All chemical reactions need a certain amount of
activation energy to get them started. Even exothermic reactions need
activation energy to get started. Once a few molecules react, the rest will
quickly follow, because the first few reactions provide the activation energy
for more molecules to react. Endothermic reactions not only need activation
to get started. They also need additional energy from the environment to
keep going.
Chemical reactions don’t all occur at the same rate. How fast a reaction
happens depends on how often and with how much energy the particles of
the reactants come together. Chemists can control rates of reactions by
changing factors such as surface area, temperature, and concentration, and
by using substances called catalysts and inhibitors.
A third way to increase the rate of a reaction is to increase the
concentration of the reactants. The concentration is the amount of a
substance in a given volume. Increasing the concentration of reactants
makes more particles available to react.
When a solid reacts with a liquid or a gas, only the particles on the
surface of the solid come in contact with the other reactant. To increase the
rate of reaction, you can break the solid into smaller pieces that have more
surface area. More material is exposed, so the reaction happens faster.
Another way to increase the rate of a reaction is to increase its
temperature. When you heat something, its particles move faster. Fastermoving particles come into contact more often, which means there are more
opportunities for a reaction to occur. Faster-moving particles also have more
energy. This energy helps the reactants get over the activation energy
“hump.”
Another way to control the rate of a reaction is to change the activation
energy needed. If you decrease the activation energy, the reaction happens
faster. A catalyst is a material that increases the rate of a reaction by lowering
the activation energy. Catalysts affect the reaction rate, but they are not
considered reactants. The cells in your body contain biological catalysts,
called enzymes. Enzymes increase the reaction rates of chemical reactions
necessary for life.
Sometimes a reaction is more useful when it can be slowed down rather
than speeded up. A material used to decrease the rate of a reaction is called
an inhibitor. Most inhibitors work by preventing reactants from coming
together.
Name ____________________________ Date ____________________ Class ____________
The Work of Scientists
■
Section Summary
Safety in the Science Laboratory
Guide for Reading
■ Why is preparation important when carrying out scientific investigations
in the lab and in the field?
■
What should you do if an accident occurs?
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The Work of Scientists
Good preparation helps you stay safe when doing science activities in the
laboratory. Preparing for a lab should begin the day before you will perform
the lab. It is important to read through the procedure carefully and make
sure you understand all the directions. Also, review the general safety
guidelines in Appendix A. The most important safety rule is simple: Always
follow your teacher’s instructions and the textbook directions exactly. Labs
and activities in this textbook include safety symbols. These symbols alert
you to possible dangers in performing the lab and remind you to work
carefully. The symbols are explained in Appendix A. When you have
completed the lab, be sure to clean up the work area. Follow your teacher ’s
instructions about proper disposal of wastes. Finally, be sure to wash your
hands thoroughly after working in the laboratory.
Some investigations will be done in the “field.” The field can be any
outdoor area, such as a schoolyard, a forest, a park, or a beach. Just as in the
laboratory, good preparation helps you stay safe when doing science
activities in the field. There can be many potential safety hazards outdoors,
including severe weather, traffic, wild animals, or poisonous plants.
Advance planning may help you avoid some potential hazards. Whenever
you do field work, always tell an adult where you will be. Never carry out a
field investigation alone.
At some point, an accident can occur in the lab. When any accident
occurs, no matter how minor, notify your teacher immediately. Then,
listen to your teacher’s directions and carry them out quickly. Make sure
you know the location and proper use of all the emergency equipment in
your lab room. Knowing safety and first aid procedures beforehand will
prepare you to handle accidents properly.