Postulates of Kinetic Theory of Gas

In order to explain the kinetic and molecular behaviour of the gaseous molecules, a theoretic model was proposed, which is known as kinetic molecular theory of gases or simply kinetic theory of gases. It was put forward by Bernoulli in 1738 and was further extended by Clausius (1857), Maxwell (1860) and Boltzmann. This model is also known as microscopic model because it is related to microscopic particles.

The main postulates of this theory are summarized as below:

• All gases are made up a very large number of small particles called molecules.

• Gas molecules are separated from one another by a large distance. That is, the intermolecular space between the gaseous molecules is large.

• The molecules are so small that the actual volume occupied by the individual molecule can be neglected as compared to the total volume of the gas. There is a large empty space between gaseous molecules. Therefore, there is no force of attraction or repulsion between the molecules. The molecules move completely independent of one another and laws of motion can be applied to them.

• Gaseous molecules are highly compressible in nature. Volume of a gas can be compresed by increasing pressure. 

• Molecular collision in a gas is perfectly elastic, i.e, there is no loss of energy when the molecules collide with each other.

• The molecules are in a state of rapid motion in all directions, a molecule always continues its straight course unless it collides with the wall of the vessel or strikes with another molecules.

• Gases exert pressure when the molecules bombard against the wall of the vessel. 

• The average kinetic energy of the gaseous molecules is directly proportional to absolute temperature (KE ∝ T).

• There is no effect of gravity on the gaseous molecules. This is the reason for whether heavier gases do not form the lower layer on the mixture. Instead, gases form homogenous mixture due to diffusion.