5.10 understand that molecules in a gas have a random motion and that they exert a force and hence a pressure on the walls of a container

Particle theory suggests that gas molecules has a random motion. When gas molecules collide with something (could be anything as they move in a random motion) they exert a force. If the gas is present in a sealed container they will exert an outward force should they hit the walls of the container.

NOTE: The pressure exerted is not the same for every gas/molecule, it will depend on how fast the particles are going and how often (as they will have more/less kinetic energy. The overall pressure felt on the object will depend on how fast the molecules are going and how often gas particles collide with its walls (more collisions = more force).