Q. Explain why does heated air expand?

Answer:-

Temperature is habitually defined as the average thermal energy of the substance at hand. Therefore the hotter a substance is the additional thermal energy it has. For the majority substances the statistical mechanics formula (3/2) kT works fine. In the previous formula k is the Boltzmann constant and T is the absolute temperature.

As thermal energy enlarge the molecules of a substance move faster. These molecules strike other molecules faster and harder and bounce away from collisions faster and harder. With no container to limit its shape a gas will merely expand. Were a container show the pressure of the gas in the container would increase.