in order to explain the reason behind the experimental behaviors of gasses. the kinetic theory is introduced. in this theory the following points are assumed known as postulates of kinetic theory. 1 gasses are composed of extremely small particles called molecules. 2 the molecules of a gas are identical in their size and masses but differ from the other gasses. 3 these molecules move at randomly in all the possible directions with any possible velocity ranging from. 4 the volume of a molecule is extremely small and neglected. 5 during the random motion the molecules collide with each other and also with the wall of container. 6 the collisions are perfectly elastic therefore there will not be loss or gain of kinetic energy. 7 the average distance travel by a molecule between two successive collisions is called mean free path L. 8 the time during which the collision lasts each neglected in comparison to the time taken to travel the molecules for collision. 9 the pressure of a gas is due to collision of a molecules with the wall of container. 10 in fact the pressure exacted by a gas is "the change in momentum per second for unit area of the container".