Liquid oxygen (also LOx, LOX or Lox in the aerospace industry) is the liquid form of oxygen. It has a pale blue colour and is strongly paramagnetic. Liquid oxygen has a density of 1140 kg/m³ and is moderately cryogenic (freezing point: −219 °C, boiling point: −183 °C). Oxygen is found naturally in the air. For industrial applications it is obtained from air by fractional distillation.
Liquid oxygen is a powerful oxidising agent: organic materials will burn rapidly and energetically in liquid oxygen, hence LOx is a common liquid oxidizer propellant for spacecraft rocket applications usually in combination with liquid hydrogen or kerosene. It was used in the very first rocket applications like the V2 missile and Redstone, R-7 or Atlas boosters. LOX is useful in this role because it creates a high specific impulse. LOx was also used in some early ICBMs although more modern ICBMs do not use LOX because its cryogenic properties and need for regular replenishment to replace boiloff make it harder to maintain and launch quickly.
Liquid oxygen sold under the brand name "Odorox" has an added synthetic odorant (dimethyl sulfide ) as a safety feature. If a user can smell the odorant, the oxygen has been evaporating and mixing with air, possibly raising the ambient oxygen concentration to a level that greatly increases the risk of fire.
Liquid nitrogen has a significantly lower boiling point (77 K) than oxygen (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.
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