Equation of state

An equation of state is an equation which relates the variables of state (T, P, V, and n). It's particularly useful when you want to know the effect of a change in one of the variables of state

1. Solids and Liquids

If the pressure on a solid or liquid is increased, the volume does not change much. If the temperature is increased, the volume doesn't change much either. Therefore, an appropriate equation of state describing such systems would be: V(T,P) = constant.

2. Gases

In contrast, changing the pressure or temperature of a gas will have an easily observable effect on the volume of that gas. For an ideal gas (no intermolecular interactions and no molecular volume) n appropriate equation of state would be: V(T,P,n) = (nRT)/P.

There are many equations of state describing real gases. These equations take in consideration molecular volume and interactions. The most well-known such equations is probably the Van der Waals equation.