If a helium-filled balloon initially at room temperature is placed in a freezer, will its volume increase, decrease, or remain the same? 3. The attempt at a solution If you decrease the T of a vessel filled with gas, its molecules will have less kinetic energy, and the pressure inside the vessel will decrease. Since He is an ideal gas, PV=nRT. So, as both T and P decrease (I assume that in the same proportion), there is no reason for that the volume should decrease in order to keep PV/T constant. But it happens, why?