What Is Liquid Air and Where Is It Used?
Liquid air is ordinary air—a mixture of oxygen and nitrogen—compressed until it forms a liquid. When the pressure on the liquid is released, it turns back into these two gases. The liquid is kept under great pressure in strong steel cylinders. In commerce, liquid oxygen is classified as an industrial gas and is widely used for industrial and medical purposes. Liquid oxygen is obtained from the oxygen found naturally in airby fractional distillation in a cryogenic air separation plant.
Liquid oxygen—abbreviated LOx, LOX or Lox in the aerospace, submarine and gas industries—has a great many uses, because it enables large amounts of these two gases to be stored in small spaces. The aqualung consists of small cylinders of liquid air joined to a mouthpiece. Liquid air is also used by pilots, firemen and astronauts. Once liquid air has been made, the liquid oxygen can be separated from it for use in hospitals. People with lung trouble are helped by being given an increased amount of oxygen to breathe. Other gases, liquefied in the same way, are used as anesthetics to put people to sleep during operations and surgeries.
Liquid oxygen is widely used in industry because a flame fed with additional oxygen is much hotter than an ordinary flame. The oxy-acetylene torch is connected to two cylinders, one of liquid oxygen and one of acetylene. The two gases are mixed together and fed through tubes to the torch. When the gases are lit, they can produce a flame with a temperature of about 3,000 °C. (5,432 °F) or twice the melting point of iron. This torch is used to cut and to weld metals.
Because of its cryogenic nature, liquid oxygen can cause the materials it touches to become extremely brittle. Liquid oxygen is also a very powerful oxidizing agent: organic materials will burn rapidly and energetically in liquid oxygen. Further, if soaked in liquid oxygen, some materials such as coal briquettes, carbon black, etc., can detonate unpredictably from sources of ignition such as flames, sparks or impact from light blows. Petrochemicals, including asphalt, often exhibit this behavior.
The tetraoxygen molecule (O4) was first predicted in 1924 by Gilbert N. Lewis, who proposed it to explain why liquid oxygen defied Curie’s law. Modern computer simulations indicate that although there are no stable O4 molecules in liquid oxygen, O2 molecules do tend to associate in pairs with antiparallel spins, forming transient O4 units.
Liquid nitrogen has a lower boiling point at −196 °C (77 K) than oxygen’s −183 °C (90 K), and vessels containing liquid nitrogen can condense oxygen from air: when most of the nitrogen has evaporated from such a vessel there is a risk that liquid oxygen remaining can react violently with organic material. Conversely, liquid nitrogen or liquid air can be oxygen-enriched by letting it stand in open air; atmospheric oxygen dissolves in it, while nitrogen evaporates preferentially.