Liquefaction of gases is the physical conversion of a gas into a liquid state. When pressure on a gas is increased, its molecules closer together, and its temperature is reduced, which removes enough energy to make it change from the gaseous to the liquid state. Ammonia gas can be liquefied by applying high pressure and lowering the temperature. Any gas can be liquefied when the gas has been exposed to the high pressures and low temperatures. The pressure that has to be applied as well as the temperature depends on the molecular forces of the gas molecules.