A substance in the gaseous state of matter is one which has no definite shape or volume. The particles in a gas are moving with high energies and speeds. There are many familiar properties of gases. For example, a gas will take on the shape of any container in which it is placed. Likewise, a gas will occupy any volume which is made available to it. A gas can also be easily compressed when pressure is exerted on it. All gases show the same physical properties. They all have mass, they can move through each other, they exert pressure, and the pressure of all gases depends on their temperature.

The physical properties of gases are explained by a model called the kinetic-molecular theory of gases. This theory is composed of several postulates. First of all, a gas is made up of tiny particles. These tiny particles...