The gas laws are strictly valid for an ideal gas under all conditions of temperature and pressure. Real gases show deviations from these laws at low temperature and high pressure.

Gases deviate from ideal behavior due to the following faulty assumptions of kinetic theory:

  1. Volume of the molecules of a gas is negligibly small in comparison to the space occupied by the gas.
  2. There is no force of attraction between the molecules of a gas.

The volume occupied by the molecules of a gas becomes significant at high pressures. If nb is the volume occupied by the molecules, the actual volume of the gas is (V – nb).

At high pressures, molecular interactions start operating. Molecules are dragged back by other molecules which affects the pressure exerted by them on the walls of the container.

Pideal = Preal + an2/V2

In view of the corrections for pressure and volume, ideal gas equation can be rewritten as:


This is known as van der Waals equation.