1. business and industrial
2. energy
3. natural gas

SECTION 10.1
The Kinetic-Molecular
Theory of Matter
Teacher Notes and Answers
2.Real gases approach ideal gas behavior at high
SECTION 1 The Kinetic-Molecular Theory of
Matter
3.​H2​ ​O, N​H​3​, and HCl would deviate from ideal
1.The kinetic energy of a single particle in a gas is
always greater than zero.
2.faster
3.Real gas particles feel some slight attractive
forces from other particles. In an ideal gas, the
particles move randomly and independently.
Review
1.An ideal gas is a hypothetical gas that fits
temperatures and low pressures.
behavior because they are polar.
4.Pressure exerted by gases is caused by collisions
of the gas molecules with a surface.
5.Gas particles are pushed closer together.
6.Gas particles move more rapidly, increasing their
average kinetic energy.
7.The lower mass of hydrogen molecules means
that they are traveling at higher speeds, so it is
easier for them to escape the force of gravity.
perfectly all the assumptions of the kinetic
theory.
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SECTION 10.1
The Kinetic-Molecular
Theory of Matter
In the late nineteenth century, scientists developed a theory
to account for the particles that make up matter. This theory
explains the differences between the three states of matter:
solid, liquid, and gas.
The kinetic-molecular theory is based on the idea that
particles of matter are always in motion. The properties of
solids, liquids, and gases are a result of the energy of their
particles and forces acting between the particles.
Key Terms
kinetic-molecular theory
ideal gas
elastic collision
diffusion
effusion
real gas
The kinetic-molecular theory explains the constant
motion of gas particles.
The kinetic-molecular theory provides a model of what is
called an ideal gas. An ideal gas is a theoretical gas that
perfectly fits these five assumptions of the kinetic-molecular
theory.
1.
Gases consist of large numbers of tiny particles that are
far apart relative to their size. Most of the volume
occupied by a gas is empty space.
2.
Collisions between particles or a particle and a container
are elastic. In an elastic collision no kinetic energy is lost,
but energy may be transferred between particles. A gas
exerts pressure on its container through the collisions of
its particles with the container.
3.
Gas particles are always in motion, so they always have
kinetic energy. They can move freely in all directions.
4.
There are no forces of attraction between particles. They
behave like billiard balls. When they collide they immediately bounce apart, instead of sticking together.
5.
The temperature of a gas depends on the average kinetic
energy of its particles. If the temperature of a gas
increases, the average speed of its particles increases. If it
decreases, the average speed of its particles decreases.
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C H A P TER 1 0
Gas particles travel in straight lines
until they collide with each other or
a container wall.
READING CHECK
1. What is always true about the
kinetic energy of a single particle
in a gas?
Kinetic Energy and Temperature The kinetic energy of any
moving object is given by the following equation.
KE = __
​ 1 ​  m​v2​ ​
2
The quantity m is the mass of the particle and v is the speed of
the particle. In a specific gas, the mass of each particle is the
same, so the kinetic energy of the gas, and thus its
temperature, depends only on the speed of the particles.
READING CHECK
2. If the temperature of a gas
increases, then its particles move
.
In a mixture of gases, the particles of each gas have the
same temperature and the average kinetic energy. Consider
such a mixture of hydrogen gas and oxygen gas. The hydrogen
particles are moving much faster on average than the oxygen
particles because they have less mass.
The kinetic-molecular theory explains the constant
motion of gas particles.
(a)
The kinetic-molecular theory states that particles in an ideal
gas are always moving. This section will describe how the
theory explains the physical properties of an ideal gas.
Expansion
Gases do not have a definite shape or volume. The particles
move randomly until they fill any container. A gas that enters
a container twice as large expands to fill the new container.
Fluidity
Particles in a gas feel no attractive forces, so they slide past
each other. In other words, gases flow in the same way that
liquids flow. Both gases and liquids are referred to as fluids.
(b)
(a) Gas particles expand to fill the
cylinder when the piston is raised.
(b) Lowering the piston exerts
pressure on the gas, compressing it
into a smaller volume.
Low Density
Gases have a very low density compared to liquids and solids.
Gas particles typically occupy a volume 1000 times greater
than an equal number of particles in a liquid or solid.
Compressibility
A compression is a reduction in volume. Because particles in a
gas are so far apart, the volume of a gas can be dramatically
decreased. Gases are often kept compressed in high-pressure
steel cylinders for industrial purposes. These cylinders can
hold over 100 times more gas than unpressurized cylinders.
Gas cylinders used in scuba diving
hold compressed air so that the diver
can carry more air at one time.
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final
4-12 -11
LKell
Diffusion and Effusion
Diffusion is the mixing of the particles of two
substances caused by their random motions. Because
gas particles are so spread out, two gases that are
released into a container can easily occupy the same
space. The gases will mix together just through the
natural motion of their particles; they do not require
additional stirring.
Gas particles passing through a tiny opening is
called effusion. The rate of effusion depends on the
velocities of the particles. A low-mass gas such as
hydrogen effuses through an opening more rapidly
N2
H2in a mixture, because its particles
than other gases
Stopcock
travel at higher speeds at a givenclosed
temperature.
H2
N2
Stopcock
closed
(a)
(b)
Stopcock
open
Real gases do not behave according to the
kinetic-molecular theory.
(a)
(b)
The kinetic-molecular theory only applies to ideal
gases. However, ideal gases do not actually exist. Kineticmolecular theory is still useful because, as long as the pressure
is not too high or the temperature too low, many gases behave
like ideal gases.
A real gas is a gas that does not behave completely
according to the assumptions of kinetic-molecular theory.
Real gas particles feel some attractive forces from other
particles. These effects are minor, unless the temperature is
low or the pressure is high. Then the particles are either too
close together or do not have enough energy to escape the
influence of attractive forces.
Noble gases such as helium and neon behave more like
ideal gases because their particles have little attraction for
each other. These gases consist of monatomic particles, which
are neutrally charged and stable.
For the same reason, nonpolar gases such as hydrogen
and nitrogen also behave like ideal gases. Gases with polar
molecules, such as water vapor and ammonia, deviate the most
from ideal gas behavior because of attractive forces between
the molecules.
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C H A P TER 1 0
Gases diffuse readily into one another.
When the stopcock is open, both gases
act as if they are occupying an
identical, larger container and spread
out to fill the entire volume together.
READING CHECK
3. What is the difference between
an ideal gas and a real gas?
SECTION 10.1 REVIEW
VOCABULARY
1. What is an ideal gas?
REVIEW
2. Describe the conditions under which a real gas is most likely to
behave ideally.
3. Which of the following gases would you expect to deviate significantly from
​ ​2​, HCl, or N​H3​ ​?
ideal behavior: He, O
​ ​2​, ​H​2​, ​H​2​O, N
4. How does kinetic-molecular theory explain the pressure exerted by gases?
5. What happens to gas particles when a gas is compressed?
6. What happens to gas particles when a gas is heated?
Critical Thinking
7. DRAWING CONCLUSIONS Molecules of hydrogen escape from Earth, but
molecules of oxygen and nitrogen are held to the surface and remain in the
atmosphere. Explain.
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