If we make several plausible assumptions, the macroscopic properties of a gas can be expressed simply in terms of three gas laws. The assumptions are:

Under these conditions a gas will obey three simple laws.

The Pressure Law: The pressureof a fixed mass of gas increases linearly with temperatureat constant volume. This can be expressed as

Charles's Law: The volumeof a fixed mass of gas increases linearly with temperatureat constant pressure. This can be written as

Boyle's Law: The pressureof a fixed mass of gas is inversely proportional to the volumeat constant temperature. This can be expressed as