Gay-Lussac’s Law – Animation
At constant pressure the volume of a gas is directly proportional to temperature, is what the Gay-Lussac Law advocates.
There is an increase in temperature (notice the size of the flame and the temperature indicator) and a proportional increase in volume. As the gas mass remains constant, ie there is no gas inlet or outlet inside the cylinder (particles), the pressure also remains constant. The increase in volume is a consequence of increased agitation of the gas molecules due to the transfer of energy.
If an ideal gas: P.V = n.R.T. If the temperature rises and the pressure remains constant then the volume should increase proportionally.