Panel 1: What is a gas?

A gas is a collection of molecules which move about freely. In a gas, the distance a molecule travels between one collision (either with another gas molecule or the wall of the container) is large compared to the size of the molecules. Because the molecules of a gas are relatively far apart, the physical properties of a gas do not depend much on the particular element or compound whose molecules make up the gas. It is reasonable to treat the molecules as simply very small, hard spheres (or points) which move about rapidly, colliding occasionally with the walls of the container or each other.