Liquefaction of Gases

Any gas can be liquefied at atmospheric pressure if it is cooled sufficiently. Many gases (but not all) can be liquefied at ordinary temperature by compressing them. The conditions of temperature and pressure under which gases liquefy were first investigated by Andrews in 1869.

Andrews subjected CO₂ to various pressures at different temperatures and plotted the effect of pressure on volume. The curve obtained at a given temperature is called an isotherm. At 321 K, the volume of the gas decreased with the increased pressure approximately in accordance with the Boyle’s Law. At 294 K, however, the volume first decreases in accordance with Boyle’s Law until the pressure was increased to about 60 atm. At this pressure there was a sudden break in the curve and liquid carbon dioxide appeared. The pressure remained constant until all the gas had been converted into liquid. Subsequent increase of pressure caused practically no change in volume.

Similar changes took place when the isotherms were constructed for temperatures below 294 K, except that the pressure required to liquefy the gas became smaller as the temperature decreased. Andrews found that liquefaction could be brought about at all temperatures below 304.1 K. But, above this temperature no liquefaction occurred no matter how much pressure was increased. This temperature was called the critical temperature for CO₂.

The pressure required to liquefy the gas at the critical temperature was called critical pressure, and the volume of 1 mole of the substance at the critical temperature and pressure, the critical volume.

The temperature above which a gas cannot be liquefied, however large the pressure may be is known as critical temperature.