Question

What are the necessary conditions for liquification of gases?

Answer 1

Critical temperature
Critical pressure
critical temperature of a gas is the temperature at or above which no amount of pressure, however great, will cause the gas to liquefy. The minimum pressure required to liquefy the gas at the critical temperature is called the critical pressure.

Answer 2

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◆Liquefaction of gases is the process by which a gas is converted to a liquid. For example, oxygen normally occurs as a gas. However, by applying sufficient amounts of pressure and by reducing the temperature by a sufficient amount, oxygen can be converted to a liquid◆

◆Liquefaction is an important process commercially because substances in the liquid state take up much less room than they do in their gaseous state. As an example, oxygen is often used in space vehicles to burn the fuel on which they operate. If the oxygen had to be carried in its gaseous form, a space vehicle would have to be thousands of times larger than anything that could possibly fly. In its liquid state, however, the oxygen can easily fit into a space vehicle's structure.
Liquefaction of a gas occurs when its molecules are pushed closer together. The molecules of any gas are relatively far apart from each other, while the molecules of a liquid are relatively close together. Gas molecules can be squeezed together by one of two methods: by increasing the pressure on the gas or by lowering the temperature of the gas◆

◆Critical temperature and pressure◆

◆Two key properties of gases are important in developing methods for their liquefaction: critical temperature and critical pressure. The critical temperature of a gas is the temperature at or above which no amount of pressure, however great, will cause the gas to liquefy. The minimum pressure required to liquefy the gas at the critical temperature is called the critical pressure◆

◆For example, the critical temperature for carbon dioxide is 88°F (31°C). That means that no amount of pressure applied to a sample of carbon dioxide gas at or above 88°F will cause the gas to liquefy. At or below that temperature, however, the gas can be liquefied provided sufficient pressure is applied. The corresponding critical pressure for carbon dioxide at 88°F is 72.9 atmospheres. In other words, the application of a pressure of 72.9 atmospheres on a sample of carbon dioxide gas at 88°F will cause the gas to liquefy. (An atmosphere is a unit of pressure equal to the pressure of the air at sea level, or approximately 14.7 pounds per square inch.)◆

◆A difference in critical temperatures among gases means that some gases are easier to liquefy than are others. The critical temperature of carbon dioxide is high enough so that it can be liquefied relatively easily at or near room temperature. By comparison, the critical temperature of nitrogen gas is −233°F (−147°C) and that of helium is −450°F (−268°C). Liquefying gases such as nitrogen and helium present much greater difficulties than does the liquefaction of carbon dioxide◆

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