Liquefaction of Gases

In general, gases can be liquefied by one of three general methods: (1) by compressing the gas at temperatures less than its critical temperature; (2) by making the gas do some kind of work against an external force, causing the gas to lose energy and change to the liquid state; and (3) by making gas do work against its own internal forces, also causing it to lose energy and liquefy.

In the first approach, the application of pressure alone is sufficient to cause a gas to change to a liquid. For example, ammonia has a critical temperature of 406K (271.4°F [133°C]). This temperature is well above room temperature, so it is relatively simple to convert ammonia gas to the liquid state simply by applying sufficient pressure. At its critical temperature, that pressure is 112.5 atmospheres, although the cooler the gas is to begin with, the less pressure is needed to make it condense.