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Assumptions stated by Classius

All the molecules of a gas are identical with respect to their shape and mass. The molecules of different gases are different.

The gas molecules behave as rigid, elastic and smooth spheres.

The size of gas molecules is very less when compared to the intermolecular distance between them. Therefore, the volume occupied by the molecules is negligible when compared to the volume of the gas.

The molecules of a gas are in a state of continuous random motion. They move with all possible velocities in all possible directions. They obey Newton's laws of motion.

The molecules collide with one another and with the walls of the container. These collisions are perfectly elastic i.e., there is no loss of kinetic energy in these collisions.

The number of molecules per unit volume of the gas is extremely large. In the steady state, this number remains constant. In other words, the molecular density of the gas remains unchanged throughout the gas.

Sub Topics

free path

free path

Between two successive collisions, a molecule moves with uniform velocity. The distance between any two consecutive collisions is called . The average distance traveled by a molecule between successive collisions is called the mean-free path.

The molecules do not exert any force of attraction or repulsion on each other, except during collision. Therefore, the molecules do not possess any potential energy. Their energy is wholly kinetic.

The collisions are instantaneous i.e., the time spent by a molecule in a collision is very less when compared to the time elapsed between two consecutive collisions.

Note : None of these assumptions are strictly true. However, the model developed on the basis of this model can be safely applied to all the gases.