有關 PV=nRT

The volume of a gas is proportional to its absolute temperature under a constant pressure condition.

When the temperature is raised, gas molecules move faster than before. As a result, the frequency of bombardment of molecules and hence the change in momentum per unit time with container walls increases, leading to a higher pressure on the container walls.

If the walls are not fixed (i.e. movable), then the increase in pressure would push the walls away, causing an increase in volume of the gas. In so doing, the no. of impact of gas molecules per unit time with the container walls will decrease, because molecules now need to travel a longer distance across the container (due to an increase in gas volume). The final result is that the gas volume increases as the temperature is increased with the pressure kept constant.