THE KINETIC THEORY OF GASES
Unlike the condensed phases of matter—solid and liquid—the atoms or molecules
that make up a gas do not move around relatively fixed positions. Rather, the
molecules of a gas move freely and rapidly, in a chaotic swarm.
A confined gas exerts a force on the walls of its container because the molecules
are zipping around inside the container, striking the walls and rebounding. The
magnitude of the force per unit area is called pressure, and is denoted by P.
P =
The SI unit for pressure is the N/m^2 , the pascal (abbreviated Pa). As we’ll see, the
faster the gas molecules are moving, the more pressure they exert. Be careful not to
confuse pressure and force. Pressure is force divided by the area over which the
force acts. For example, if you placed a box weighing 1,000 N on a tabletop whose
area was 1 m^2 , then the pressure would be 1,000 pascals. But if that same box were
placed on a thin vertical column whose top had an area of only 1 cm^2 (or 10−4 m^2 ),
then the pressure would be 10,000,000 pascals. In each case, the force was the
same, but the areas over which the force acted were different, which is why the
pressures were different.
We also need a way to talk about the typically vast numbers of molecules in a given
sample of gas. One mole of atoms or molecules contains
NA = 6.022 × 10^23
of these elementary quantities. The number NA is known as Avogadro’s constant,