Pressure is the force per unit of area exerted against a surface. In gases, pressure is just as important in its effect on expansion as is heat. (Pressure has little effect in this respect in solids and liquids.) All gases expand at about the same rate when heated—provided the pressure upon the gas does not change. If the pressure varies, however, the rate of expansion also varies.

An increase of pressure raises both the melting point of most solids and the boiling point of most liquids. Exceptions are ice, bismuth, and cast iron—substances that possess the unusual property of expanding when they freeze. Their melting points (but not their boiling points) are lowered by pressure.