Why gas expands on heating?
On heating any substance the intermolecular force becomes less stronger.The heat energy is transformed into the molecules and they become energized and is converted into kinetic energy and with this energy they move away from each other and expand. This causes the solid to form liquid.Liquid to form gas and gas expands and expands until it changes into an other element or heat is not applied
Here is the equation for the kinetic energy of a gas. I think it's an ideal gas, and this is a statistical mechanics distribution derived calculations, so I think there are two others, but this is the most well known.
kinetic energy = (1 / 2) * mass * (velocity)^2 = (3 / 2)(Boltzmann's constant)(Temperature)
If you rearrange and solve for velocity, you see that as temperature increases, velocity increases, as well.
Therefore, the temperature of a substance is simply a measure of how quickly the particles are moving around.
Heat causes the molecules to move faster, (heat energy is converted to kinetic energy) which means that the volume of a gas increases more than the volume of a solid or liquid. However, gases that are contained in a fixed volume cannot expand - and so increases in temperature result in increases in pressure.