Pressure and volume of a gas

The pressure of the gas is due to the collisions of the molecules with the walls of the container. Every time there is a collision the molecules experience a change of momentum – the means that there is a force on the container walls and many collisions over the areas of the walls of the container results in a pressure. For a given gas the faster the molecules move and the more collisions per second they make with the walls of the container the greater the pressure of the gas.

The number of collisions per second can be changed in two ways:

(a) changing their speed so that each change of momentum on collision is less:
(i) making them move faster – heating the gas
(ii) making them move more slowly – cooling the gas

(b) changing the volume of the container so that they collide with the walls less often
(i) expanding the container so that they take more time between collisions
(ii) contracting the container so that they collide more often with the walls

So expanding the container will make the molecules collide with the walls less often and so the pressure of the gas will drop.