why do gases show deviation from ideal behaviour?
Reasons for deviation from the ideal gas behaviour is the two postulates of the kinetic theory which do not hold good which are:
There is no force of attraction and repulsion between the molecules of a gas.
Volume of the molecules of a gas is negligibly small in comparison to the space occupied by the gas.
Real gas deviates from ideal gas behaviour as there are forces of attraction and repulsion between the molecules of a gas. At high pressure, while striking the walls of a container, the molecules of a gas are dragged back by other molecules due to the existing force of attraction; therefore, the pressure exerted by the gas is lower than the pressure exerted by an ideal gas.
Real gas deviates from ideal gas behaviour as the volume occupied by the gas molecules becomes significant. This is because the movement of the gas particles is restricted to the volume (V − nb), instead of V. Here, nb is the approximate total volume of the molecules themselves, excluding the spaces between them.
This means, Videal = Vreal − nb