Oxygen as a gaseous element forms 21% (by volume) of the earth’s atmosphere. Oxygen is a liquid below its boiling point of -183°C (-297°F). Liquid oxygen is obtained by the liquefaction of air. It is separated from the other liquid atmospheric elements by
fractional distillation.
The development of satisfactory containers for storage of cryogenic liquids at very low temperatures has enabled air liquefaction products to be transported and stored on the premises of the ultimate user. As a liquid, 4.0 L (1.06 gallons) of oxygen will produce 25.2 m3 (890 ft3) of gaseous oxygen at 21°C (70°F) and 101.4 kPa (14.7 psia).
Liquid oxygen is converted to a gas at the job site, providing a source of high purity oxygen for welding, cutting, and other operations in which large volumes of gases are used. In addition to welding and industrial uses, liquid oxygen has an important role as a fuel for rocket engines in the United States space program. High purity oxygen is extensively used for oxygen therapy in hospitals and medical service organizations.
See OXYGEN PRODUCTION; LIQUID AIR PROCESS, and CRYOGENICS.