Causes of Deviation from Ideal Behaviour

Experimentally it has been observed that the real gases obey the gas laws or the Ideal gas equation only if the pressure is low or the temperature is high. However, if the pressure is high or temperature is low, the real gases show marked deviation from Ideal behaviour. The reason for such a deviation shown by real gases has been found to be as follows:

The derivation of the gas laws and hence Ideal gas equation is based upon the Kinetic theory of gases which in turn is based upon certain assumptions. A careful study shows that at high pressure or low temperature, the following two postulates of Kinetic theory of gases are faulty;

(1).  The volume occupied by the gas molecules is negligible as compared to the total volume of gas.

(2).  The forces of attraction or repulsion between the gas molecules are negligible.

The above two assumption are true only if the pressure is low or the temperature is high so that the distance between the molecules is low, the gas molecules come close together. Hence under these conditions;

¤    The forces of attraction or repulsion between the molecules may not be negligible.

¤    The volume occupied by the gas may be so small that the volume occupied by the molecules may not be negligible.