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Explaining the Behavior of Gases The kinetic-molecular theory helps explain the behavior of gases.CROSSCUTTING CONCEPTS Systems and System Models Review the assumptions made by the kinetic-molec- ular theory.For example, the constant motion of gas particles allows a gas to expand until it fills its container, such as when you inflate a beach ball.Thus, there are fewer chlorine molecules than gold atoms in the same volume.Develop your own visual or physical representation to help others understand the kinetic-molecular theory and the behavior of gases.Compression and expansion If you squeeze a pillow made of foam, you can compress it; that is, you can reduce its volume.When the volume of a container is made larger, the random motion of the particles fills the available space.Figure 3 illustrates what happens to the density of a gas in a container as it is compressed and as it is allowed to expand.Air, which is a mixture of gases, is also compressible.

Explaining the Behavior of Gases
The kinetic-molecular theory helps explain the behavior of gases. For example, the
constant motion of gas particles allows a gas to expand until it fills its container, such as
when you inflate a beach ball. As you blow air into the ball, the air particles spread out
and fill the inside of the container—the beach ball.
Low density
Remember that density is mass per unit volume. The density of chlorine gas is
2.898 × 10–3 g/mL at 20°C; the density of solid gold is 19.3 g/mL. Gold is more than
6700 times as dense as chlorine. This large difference cannot be due only to the
difference in mass between gold atoms and chlorine molecules (about 3:1). As the
kinetic-molecular theory states, a great deal of space exists between gas particles. Thus,
there are fewer chlorine molecules than gold atoms in the same volume.
Compression and expansion
If you squeeze a pillow made of foam, you can compress it; that is, you can reduce its
volume. Air, which is a mixture of gases, is also compressible. The large amount of
empty space between the particles in the air allows air to be squeezed into a smaller
volume. When the volume of a container is made larger, the random motion of the
particles fills the available space. Figure 3 illustrates what happens to the density of a
gas in a container as it is compressed and as it is allowed to expand.
Compression Expansion
Figure 3 In a closed container, compression and expansion change the volume occupied by a constant
mass of particles.
Relate the change in volume to the density of the gas particles in each cylinder.
CROSSCUTTING CONCEPTS
Systems and System Models Review the assumptions made by the kinetic-molec-
ular theory. Develop your own visual or physical representation to help others
understand the kinetic-molecular theory and the behavior of gases. What are the
limitations of your model?
WORD ORIGINS
gas
comes from the Latin word chaos
,
which means space