137.

Why do real gases show deviation from ideal behaviour?

Real gases deviate from ideal behaviour. The disobedience of the ideal gas equation, PV = n RT by real gases may be due to the following two faulty assumptions of kinetic theory of gases:

(i) The actual volume of molecules is negligible in comparison to the total volume of the gas. This assumption is true only at low pressure and high temperature. But at high pressures or at low temperatures, the total volume of the gas decreases while the volume of the molecules being incompressible remains the same. Thus, under these conditions, the volume of the molecules can not be neglected.
 
(ii) The forces of attraction or repulsion between the gas molecules are negligible. No doubt, this assumption is true only at low pressure and high temperature since the molecules lie far apart from one another. But at high pressure and low temperature, the volume of the gas is quite small. Gas molecules come closer to each other. The attractive forces between the gas molecules under these conditions are quite appreciable and cannot be neglected.