Before air is liquefied it must first be freed from dust, moisture, and carbon dioxide which, at the low temperatures used, would solidify and thus obstruct the expansion valves. The remaining gas is compressed and allowed to expand in a series of three or four cycles, the pressure becoming successively greater. The cooling effect of the final expansion becomes great enough to lower the temperature to —191 °C., at which point the gas begins to change into a liquid. Liquid air thus made may be separated into oxygen and nitrogen by fractional distillation, nitrogen (bp — 195.8 °C.) boiling off first and leaving liquid oxygen (bp —182.9 °C.). Both oxygen and nitrogen are prepared commercially in this way. Liquid air may also be used to produce high vacua, to maintain very low temperatures and to dry and purify gases.
Liquid air is considerably richer in oxygen content than is gaseous air. It is a nonconductor of electricity, supports the combustion of fuels immersed in it, and has a pale blue color. A coil of lead wire immersed in liquid air becomes elastic, due to the low temperature. Liquid oxygen is strongly magnetic. It is used with fuels to propel ROCKETS and rocket bombs.
Hydrogen (bp -252.7 °C.) and helium (bp -269 °C.) are the gases most difficult to liquefy. The former yields the lightest known liquid, its specific gravity being only 0.07. Many gases are liquefied before being marketed because liquids are easier to handle and occupy less space than gases. Chlorine, ammonia, sulfur dioxide, and carbon dioxide are among the commercial gases sold in liquid form.
Hydrocarbon gases from oil wells, as well as those produced in the cracking of petroleum, may be liquefied and sold in steel cylinders, for heating and lighting to residents who do not have access to natural or manufactured gas.
Other liquefied hydrocarbon gases, such as ACETYLENE, are important industrial fuels. The vaporization of some liquefied gases, such as ammonia, sulfur dioxide, METHYL CHLORIDE, and FREON, produces the cooling effect used in many refrigerators.
