Kinetic theory of gases is the concept of free motion of gas molecules. It explains about the microscopic properties of the gases. Various gas laws can be easily deduced from the kinetic theory of gases. The kinetic theory of gases was given by D. Bernoulli in 1738 and further developed by Joule, Clausius, Maxwell, Boltzmann, etc.
The postulates of kinetic theory of gases are as follows:
1. All the gases consist of a large number of very small particles, called molecules.
2. The molecules of a gas are identical but they are different from the molecules of other gases.
3. The gas molecules are assumed to be solid, spherical and perfectly elastic.
4. The volume occupied by the gas molecules is negligible as compared to the total volume of the gas.
5. The gas molecules are very small in size as compared to the distance between them.
6. The molecules are in continuos random motion in a straight line with velocities in all directions ranging from zero to infinity.
7. The gas molecules collide with each other and with the walls of the containing vessel. The collisions of the molecules are perfectly elastic i.e. there is no loss of energy during collisions.
8. The gas exert pressure due to the collisions of the molecules on the walls of the container.
9. A molecule moves in a straight line between two collisions. The distance covered between two successive collisions is called the free path of the molecule.
10. The collisions are almost instantaneous i.e. the collision time of a gas molecule is negligible as compared to the time taken by the molecule to cover the free path.
11. The force of attraction and repulsion between two molecules is negligible.
12. The gravitational force on the gas molecules is negligible. There is no effect of gravity during the diffusion of the gases.
13. The molecules do not exert any force on each other except during collision.
14. In a gas, not all the molecules have the same speed. So, different molecules have different kinetic energy. The average kinetic energy of a gas molecule is directly proportional to the temperature of the gas in absolute scale i.e. $K.E.∝T$.