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AHSGE Review
J. Pollock
Spring 2007
Science Skills and Chemistry
Concepts
What is the scientific method?
 It is a framework (guidelines) for
investigating a problem.
 There are five steps.
–
–
–
–
–
State the problem.
Make an educated guess, or hypothesis.
Design and conduct an experiment to test.
Analyze the data collected during the test.
State the conclusion.
What skills are necessary in order
to interpret data?
 To determine how data should be classified
and interpreted, graphs and tables are used.
 Measurements are compared to standards.
 Hypotheses are tested and either accepted as
theories or rejected.
What are controls and variables?
 Variables are parts of an experiment that can
be measured.
– They CAN change.
 Controls are parts of the experiment that do
no undergo change.
– Controls are used for comparison.
– Since the control does not change, the variable
can be compared to it and the change measured.
How is the metric system used to
measure dimension and volume?
 The metric system is measurement based on
powers of 10.
 It has standards for length, mass, time,
electric current, temperature, amount of
substance, and luminous intensity.
 Prefixes are used for very large or very
small numbers.
 Conversions can be made by moving
decimals.
Common Metric Units
Quantity
Unit
Symbol
Length
Meter
m
Mass
Kilogram
kg
Time
Second
s
Electric current
Ampere
A
Temperature
Kelvin
K
Amount of substance
Mole
mol
Luminous intensity
Candela
cd
What is a hypothesis?
 A hypothesis is an educated guess.
 You use what you already know to guess
how an experiment is going to turn out.
 It must be tested with an experiment.
What glassware is necessary for
use in the laboratory?
Top row: test tube holder, Florence flask,
funnel, Erlenmeyer flask, test tube, graduated
cylinder; Bottom row: triple-beam balance,
beaker, thermometer
What is mass, and what are its
units?
 Mass is the measure of the amount of matter
in an object.
 It is measured in milligrams, grams,
kilograms, etc.
 Kilograms are the standard unit.
 Mass is NOT the same as weight. Weight is
a force that measures the force of gravity on
an object.
What units are used for
temperature in the metric system?
 Temperature is measured in degrees Celsius
or Kelvins.
 Kelvin is the standard unit for scientific
purposes.
 Calculations can be used to convert between
the two units.
– Kelvin = Celsius + 273
– Celsius = Kelvin - 273
Temperature Conversion Examples
 Convert 100C to Kelvin.
K = 100+273
K = 373
 Convert 200 K to Celsius.
C = 200-273
C = -73
What is density?
 Density is the amount of matter in a specific
volume.
 The formula is density = mass ÷ volume (d=m/v).
 Example: An object having a mass pf 8.40 g and a
volume of 4.2 cm3 will have what density?
D = m/v
D = 8.40g / 4.2 cm3
D = 2.0 g/cm3
How is the periodic table
arranged?
 The periodic table was first published in
1869 by Russian scientist Dmitri
Mendeleev.
 The first periodic table was arranged
according to the atomic mass of elements.
 Mendeleev’s table was revised by Henry
Moseley in 1911.
 Moseley’s table was arranged by atomic
number, since this number does not change.
How is the periodic table arranged?
 The periodic table has seven periods (horizontal
rows) and eighteen groups (vertical columns).
 Groups are together because the elements in them
have similar properties and react in the same
manner.
 Across periods (left to right), atomic radius (size)
decreases, ionization energy (ease of losing an
electron) increases, and electronegativity (ability
to attract electrons) increases.
What are valence electrons, and
how do they work?
 Valence electrons are found in the outer shells of
atoms.
 These are the only electrons involved in chemical
reactions.
 The most valance electrons an atom can have is 8.
This is called the octet rule.
 The number 8 is possible by gaining, losing or
sharing electrons.
 The octet rule explains why atoms tend to form
compounds.
What are valence electrons, and how
do they work?
What are valence electrons, and how
do they work?
How is the size of an atom
measured?
 Atoms are too small to be seen or measured
with the naked eye.
 Scanning electron microscopes can be used
to detect them, but we generally see the
EFFECT of the atom, not the atom itself.
 Atoms can be measured in moles. A mole is
6.02 X 1023 atoms. The mole is just a unit.
What is chemical reactivity?
 Reactivity is the ability of an atom to
undergo a reaction and the speed of the
reaction.
 It refers to the ability of the element to react
with other substances.
 The element’s position on the periodic table
determines its reactivity.
Compare and contrast metals,
nonmetals, and metalloids.
Metal
Nonmetal
Metalloid
Shiny
Dull
Dull
Hard, malleable, and
ductile
Brittle
Intermediate ability to
break
Excellent conductor of
heat and electricity
Poor conductor of heat
and electricity
Intermediate conductor
of heat and electricity
Mostly solid at room
temperature
Mostly gas at room
temperature
Mostly solid at room
temperature
Left side of periodic
table
Right side of periodic
table
Along zig-zag line of
periodic table
Compare and contrast metals,
nonmetals, and metalloids.
What are noble gases?
 The noble gases are found in group 18 of
the periodic table.
 They are generally unreactive and were not
discovered until 1962.
 They can react with other elements when
they are treated.
 Noble gases are very useful because they
are stable.
Noble Gases
What is the difference between a
family and a period?
 Families are the vertical columns, or
groups, of the periodic table.
 Periods are the horizontal rows of the
periodic table.
 Families are arranged because of similar
properties. Each element in a group will
react and appear much the same.
Coded Periodic Table
What is the atomic number and
how is it used?
 The atomic number is the number of
protons in an atom of a particular element.
 The number of protons cannot change, so
the atomic number of an element will
always remain the same.
 Atomic number is useful for organizing
elements on the periodic table. They are
listed in order of increasing atomic number.
What affects the mass number of
an atom?
 The mass number is the number of protons and
neutrons in the nucleus of an atom.
 Since the number of protons cannot change, the
number of neutrons determines the mass number
of a particular atom.
 The formula for determining mass number is # of
protons + # of neutrons = mass number.
 It is possible for an element to have several
different mass numbers.
Mass Number Calculation Example
 An atom of uranium has 92 protons and 143
neutrons. What is its mass number?
92 + 143 = 235
What are isotopes?
 Isotopes are atoms of the same element that
have different numbers of neutrons.
 The number of protons and electrons
remains the same.
 For example, carbon can be carbon-12,
carbon-13, or carbon-14. All of these
isotopes have 6 protons, but their neutrons
are 6, 7, and 8, respectively.
What are isotopes?
Isotope
Protons
Electrons
Neutrons
Carbon-12
6
6
6
Carbon-13
6
6
7
Carbon-14
6
6
8
Compare and contrast physical
and chemical changes.
 Chemical changes
– One or more
substances converted
into different
substances
– Cannot be reversed
back to original
substances by physical
means
 Physical changes
– Observed or measured
without changing the
identity of the
substance
– Melting point, boiling
point
– Phase
– Color, size, shape
What are the characteristics of
chemical changes?
 Chemical changes have specific properties.
 You can tell a chemical change has
occurred, if you observe the following.
–
–
–
–
Heat or light
Gas given off
Formation of precipitate
Color change
Compare endothermic and
exothermic reactions.
 Endothermic
– Absorbs energy
– Temperature of
surroundings lowers
– Example:
photosynthesis
– Bonds formed
 Exothermic
– releases energy
– Temperature of
surroundings rises
– Example: creation of
salt from sodium and
chlorine
– Bonds broken
– Released energy from
chemical energy
What is the difference in solute
and solvent?
 Solute is the substance in the larger quantity
in a solution. It is said to do the dissolving.
 Solvent is the substance in the smaller
quantity in a solution. It is said to be the
substance being dissolved.
 A solution is a homogeneous mixture of two
or more substances uniformly spread
throughout a single phase.
What is solubility?
 Solubility involves the amount of solute
needed to form a saturated (maximum
amount of solute dissolved) solution with a
specific amount of solvent at a certain
temperature.
 It is affected by temperature, agitation
(stirring), phase (surface area), pressure,
and polarity of the substances.
What factors affect solubility?
 Polarity
 temperature
 surface area
 stirring
 pressure
What are units of concentration?
 Units of concentration serve to show how
much solute is contained in a certain
volume of solvent.
– Molarity (moles per liter)
– molality (moles per kilogram)
How do I calculate
concentration?
 You have 3.50 L of solution that contains
1.55 mol of sodium chloride, NaCl. What is
the molarity of that solution?
 M = mol/L
 M = 1.55/3.50
 M = 0.44
How do I calculate
concentration?
 A solution was prepared by dissolving 0.05
mol of sucrose (table sugar, C12H22O11) in
0.125 kg of water. Find the molal
concentration of this solution.
 m = mol/kg
 m = 0.05/0.125
 m = 0.4
What is conductivity?
 Conductivity is the ability of a substance to
conduct heat, electricity, or sound.
 Metals and ionic compounds (acids, bases,
and salts) have high conductivity.
 Nonmetals and molecular compounds have
low conductivity.
Compare saturated, unsaturated,
and supersaturated solutions.
Saturated
Unsaturated
Contains
Less solute than
maximum of
a saturated
dissolved solute solution under
same conditions
Supersaturated
More dissolved
solute than a
saturated
solution under
same conditions
What is an electrolyte?
 An electrolyte is a substance that dissolves
in water to give a solution that conducts
electric current.
–
–
–
–
Ionic compounds
highly polar molecular compounds
strong acids
strong bases
Compare ionic and covalent
bonds.
 Ionic
– metal and nonmetal
– electrons gained or lost
– good conductor
– contains highly
electronegative
element
 Covalent
– two nonmetals
– electrons shared
– poor conductor
– similar
electronegativities
among bonded
elements
What is an electron dot diagram?
 Also known as Lewis structures (molecules)
 formulas in which atomic symbols represent
nuclei and inner-shell electrons and dots
represent outer-shell electrons
 For example
–H
He
B
Ne
How are ions used to form
bonds?
 Ions gain or lose electrons to form stable
outer shells when they bond.
 Example
Na
+
Cl  Na+ Cl
-
How are compounds named?
 Ionic compounds are named by stating the
elemental name of the cation (positive ion),
removing the ending of the cation (negative
ion), and replacing it with -ide.
 Example: sodium and chlorine
 sodium chlorine
 sodium chloride
How are compounds named?
 Molecular compounds are named using
Greek prefixes.
 Example: arsenic and oxygen (As2O5)
 diarsenic pentoxide
Greek Prefixes Used for
Compound Naming
 mono - 1
 hexa - 6
 di - 2
 hepta - 7
 tri - 3
 oct - 8
 tetra - 4
 nona - 9
 penta - 5
 deca - 10
How are chemical equations
balanced?
 There are steps for balancing chemical equations.
 First, you must list the atoms on both sides of the





equation.
Then, you must count and tally the atoms on both
sides of the equation.
Balance one type of atom at a time.
First balance the atoms of elements that appear only
once on both sides of the equation.
Balance polyatomic ions as single units.
Balance H and O last.
Balancing Equations Example
 HNO3 + Mg(OH)2  Mg(NO3)2 + H2O
 See next slide for list and tally.
 Mg is balanced.
 Balance NO3.
 2 HNO3 + Mg(OH)2  Mg(NO3)2 + H2O
 Balance OH.
 2 HNO3 + Mg(OH)2  Mg(NO3)2 + 2 H2O
 Check tally for balance.
Balancing Equations Example
21
H
21
NO3
2
NO3
1
Mg
1
Mg
2
OH
21
OH
21
H
What is the Law of Conservation
of Matter?
 The Law of Conservation of Matter states that
matter can neither be created nor destroyed in a
chemical reaction.
 This law is essential to balancing chemical
equations.
 The mass on both sides of the equation must be
equal.
 Conservation applies to mass and matter as well.
 LaVoisier proved the law completing his reactions
on a balance with a bell jar to contain the gases.
How do electrons affect chemical
reactions?
 Electrons in the outer shell ONLY are
involved in chemical reactions.
 The number of electrons being transferred
determines the properties of the substances
being combined, as well as that of the
resulting substance.
 The location of the electrons involved in the
reaction determines the amount of energy
involved in the reaction.
What are the types of chemical
reactions?
 There are five types of chemical reactions.
– Decomposition: breaking down of substances
– synthesis: creation of new substance
– single replacement: exchanging of one element
for another
– double replacement: two substances react to
form two new substances
– combustion: reaction of oxygen and a
hydrocarbon
What are the types of chemical
reactions?
 Synthesis: 2 Mg + O2  2 MgO
 Decomposition: 2 H2O  2 H2 + 2 O2
 Single Replacement: 2 N2 + 2 H2O  2
NaOH + H2
 Double Replacement: 2 KI + Pb(NO3)2 
PbI2 + 2 KNO3
 Combustion: C3H8 + 5 O2  3 CO2 + 4
H2O