A gas consists of a large number of molecules, each of which has a random motion. These molecules are very small and very close together with the scale being such that for all practical purposes a gas can be considered continuous and uniform. The behaviourof a gas is a function of the average distance between the molecules, compared to the size of molecule. If the molecule can be considered small compared to the average distance between molecules, then the potential energy arising from the mutual attraction of the molecules may be ignored and the gas can be considered an ideal or perfect gas. The important properties of an ideal gas at rest are density and pressure.
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