Liquid oxygen (LOX – LIQUID OXYGEN) is a pale blue strongly paramagnetic liquid. It is one of the four states of aggregation of oxygen. Liquid oxygen has a specific gravity of 1.141 g/cm3 and has properties such as moderate cryogenic freezing point of 50.5 K (-222.65 °C) and boiling point of 90.188 K (-182.96 °C). Liquid oxygen is widely used in the aerospace and gas industries, the operation of submarines and it is commonly used in medicine. Generally, the production of liquid oxygen is based on fractional distillation of air.
The expansion coefficient of oxygen when changing the state of aggregation into gaseous components is 860:1 at 20 °C, which is sometimes used in the oxygen supply systems for respiration in commercial and military aircrafts, as well as for transporting and supplying oxygen for long distances. One vehicle trailer carring liquid oxygen is capable of carrying as much oxygen in liquid state as oxygen in gaseous state (in 40 litre bottles) can be transported in 11 trucks!
Due to its cryogenic nature, liquid oxygen may cause brittleness in materials that come into contact with it. Liquid oxygen is also a very powerful oxidising agent: the organic material burns quickly in its environment with high heat. Moreover, some substances impregnated with liquid oxygen tend to explode unexpectedly. Petroleum products, including asphalt, often exhibit this behaviour. The use of liquid oxygen in asphalt coating is therefore not permitted.
Liquid oxygen is a common oxidiser in rocket fuel, usually in combination with liquid hydrogen or kerosene. Its use is due to high specific impulse, which is obtained by using this oxidant in rocket fuel.