It is due to two assumptions which are made in formulating kinetic molecular theory:
(i) The molecules have negligible volumes, but all real molecules have definite volumes which cannot be neglected, especially at low temperature and high pressure.
(ii) It was assumed that there is negligible force of attraction between molecules, therefore, molecules moved independently. It is not true, especially at low temperature, and high pressure when the force of attraction between molecules is appreciable.
