The causes of deviations from ideal behaviour may be due to the following two assumptions of kinetic theory of gases:
The volume occupied by gas molecules is negligibly small as compared to the volume occupied by the gas. The forces of attraction between gas molecules are negligible.
This is valid only at low pressure and high temperature, when the volume occupied by the gas molecules is negligible as compared to the total volume of the gas. Two conditions in which ideal gases show deviation:
1. At low temperature or
2. At high pressure
The volumes of molecules are no more negligible as compared to the total volume of the gas and the gas molecules interact with each other due to proximity.