Definition Of Liquefaction Of Gases

Definition Of Liquefaction Of Gases

Liquefaction, sometimes liquification, generally refers to the process of becoming a liquid or liquid-like

More About Liquefaction of Gases

  • It was found that real gases behave in the same manner as carbon dioxide
  • At high temperatures isotherms look like that of an ideal gas and the gas cannot be liquefied even at very high pressure
  • As the temperature is lowered, shape of the curve changes and data shows considerable deviation from ideal behaviour
  • At carbon dioxide remains gas upto 73 atmospheric pressure
  • At 73 atmospheric pressure, liquid carbon dioxide appears for the first time. The temperature is called critical temperature (T) of carbon dioxide
  • This is the highest temperature at which liquid carbon dioxide is observed. Above this temperature it is gas. Volume of one mole of the gas at critical temperature is called critical volume (Vc) and pressure at this temperature is called critical pressure (Pc)
  • The critical temperature, pressure and volume are called critical constants. Further increase in pressure simply compresses the liquid carbon dioxide
  • In the isotherm of carbon dioxide, the steep line represents the isotherm of liquid
  • Even a slight compression results in steep rise in pressure indicating very low compressibility of liquid
  • On further compression, liquid and gaseous carbon dioxide co exists. On still further compression, all the gas gets compressed to liquid
  • All the gases upon compression at constant temperature (isothermal compression) show the same behaviour as shown by carbon dioxide
  • Gases should be cooled below critical temperature for liquification
  • Critical temperature of a gas is highest temperature at which liquefaction of the gas first occurs
  • Liquification requires cooling as well as compression
  • Compression brings the molecules in close vicinity and cooling slows down the movement of molecules therefore, intermolecular interactions may hold the closely and slowly moving molecules together and the gas liquefies
  • It is possible to change a gas into liquid or a liquid into gas by a process in which always a single phase is present
  • If process is carried out at the critical temperature; substance always remains in one phase
  • Thus there is continuity between the gaseous and liquid state. The term fluid is used for either a liquid or a gas to recognize this continuity
  • Liquid and gas can be distinguished only when the fluid is below its critical temperature and its pressure and volume lie under the dome, since in that situation liquid and gas are in equilibrium
  • And a surface separating the two phases is visible
  • At critical temperature, liquid passes into gaseous state imperceptibly and continuously; the surface separating two phases disappears
  • A gas below the critical temperature can be liquefied by applying pressure, and is called vapor of the substance


  • The most important advantage of liquefying gases is that they can then be stored and transported in much more compact form than in the gaseous state.
  • liquid oxygen and liquid hydrogen are used in rocket engines. Liquid oxygen and liquid acetylene can be used in welding operations