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The liquefaction of gases

An increase in pressure raises the boiling point of a liquid. Many substances that are gases at normal temperatures and atmospheric pressure can be compressed to such an extent that their boiling point is raised above atmospheric temperature and the gas liquefies (e.g. propane, ammonia).

Other gases cannot be liquefied at atmospheric temperature no matter how great a pressure is applied. These are called permanent gases. However, if the temperature is lowered sufficiently, it becomes possible to liquefy them by compression (e.g. methane, oxygen).