Assumptions :

1. Gases are composed of minute particles called molecules. All the molecules of a gas are identical.

2. Gaseous molecules are always, at a random movement. The molecules are moving in all possible directions in straight lines with very high velocities. They keep on colliding against each other and against the walls of the vessel at very small intervals of time.

3. The actual volume occupied by the molecules is negligible when compared to the total volume occupied by the gas.

4. There is no appreciable attraction or repulsion between the molecules.

5. There is no loss of kinetic energy when the molecules collide with each other or with the wall of vessel. This is because the molecules are spherical and perfectly elastic in nature.

6. The pressure exerted by the gas is due to the bombardment of the molecules of the gas on the walls of the vessel.

7. The average kinetic energy of the molecules of the gas is directly proportional to the absolute temperature, Average K.E. ∝ T.

8. The force of gravity has no effect on the speed of gas molecules.