The kinetic theory of gases:

 The gas laws were empirically developed from experimental observations. The kinetic theory of gases assumes that the molecules or atoms of a gas are in continuous random motion. The pressure exerted on the walls of a containing vessel arises from the bombardment by these fast moving particles. A gas is described as a collection of particles in motion, with the macroscopic physical properties of the gas following from this premise. Pressure is regarded as the result of molecular impacts with the walls of the container, and temperature is related to the average translational energy of the molecules.

Fig.1. The Maxwell distribution of speeds for a gas, illustrating the shift in peak position and distribution broadening as the temperature increases.