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Liquid oxygen

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Liquid oxygen (O2) (light sky-blue liquid) in a beaker.
When liquid oxygen (O2) is poured from a beaker into a strong magnet, the oxygen is temporarily suspended between the magnet poles, owing to its paramagnetism.

Liquid oxygen, sometimes abbreviated as LOX or LOXygen, is a clear light sky-blue liquid form of dioxygen O2. It was used as the oxidizer in the first liquid-fueled rocket invented in 1926 by Robert H. Goddard,[1] an application which has continued to the present.

Physical properties[edit]

Liquid oxygen has a clear light sky-blue color and is strongly paramagnetic: it can be suspended between the poles of a powerful horseshoe magnet.[2] Liquid oxygen has a density of 1.141 kg/L (1.141 g/ml), slightly denser than liquid water, and is cryogenic with a freezing point of 54.36 K (−218.79 °C; −361.82 °F) and a boiling point of 90.19 K (−182.96 °C; −297.33 °F) at 1 bar (15 psi). Liquid oxygen has an expansion ratio of 1:861[3][4] and because of this, it is used in some commercial and military aircraft as a transportable source of breathing oxygen.

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.[5]

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.[6] 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.[7]

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.

The surface tension of liquid oxygen at its normal pressure boiling point is 13.2 dyn/cm.[8]


A U.S. Air Force technician transfers liquid oxygen to a Lockheed Martin C-130J Super Hercules aircraft at the Bagram Airfield, Afghanistan.

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 air by fractional distillation in a cryogenic air separation plant.

Air forces have long recognized the strategic importance of liquid oxygen, both as an oxidizer and as a supply of gaseous oxygen for breathing in hospitals and high-altitude aircraft flights. In 1985, the USAF started a program of building its own oxygen-generation facilities at all major consumption bases.[9][10]

In rocket propellant[edit]

SpaceX's liquid oxygen ball at Cape Canaveral

Liquid oxygen is the most common cryogenic liquid oxidizer propellant for spacecraft rocket applications, usually in combination with liquid hydrogen, kerosene or methane.[11][12]

Liquid oxygen was used in the first liquid fueled rocket. The World War II V-2 missile also used liquid oxygen under the name A-Stoff and Sauerstoff. In the 1950s, during the Cold War both the United States' Redstone and Atlas rockets, and the Soviet R-7 Semyorka used liquid oxygen. Later, in the 1960s and 1970s, the ascent stages of the Apollo Saturn rockets, and the Space Shuttle main engines used liquid oxygen.

In 2020, many rockets used liquid oxygen:


See also[edit]


  1. ^ "First liquid-fueled rocket". HISTORY. Retrieved 2019-03-16.
  2. ^ Moore, John W.; Stanitski, Conrad L.; Jurs, Peter C. (21 January 2009). Principles of Chemistry: The Molecular Science. Cengage Learning. pp. 297–. ISBN 978-0-495-39079-4. Retrieved 3 April 2011.
  3. ^ Cryogenic Safety. chemistry.ohio-state.edu.
  4. ^ Characteristics. Archived 2012-02-18 at the Wayback Machine. Lindecanada.com. Retrieved on 2012-07-22.
  5. ^ "Liquid Oxygen Receipt, Handling, Storage and Disposal". USAF Training Film.
  6. ^ Lewis, Gilbert N. (1924). "The Magnetism of Oxygen and the Molecule O2". Journal of the American Chemical Society. 46 (9): 2027–2032. doi:10.1021/ja01674a008.
  7. ^ Oda, Tatsuki; Alfredo Pasquarello (2004). "Noncollinear magnetism in liquid oxygen: A first-principles molecular dynamics study". Physical Review B. 70 (134402): 1–19. Bibcode:2004PhRvB..70m4402O. doi:10.1103/PhysRevB.70.134402. hdl:2297/3462. S2CID 123535786.
  8. ^ J. M. Jurns and J. W. Hartwig (2011). Liquid Oxygen Liquid Acquisition Device Bubble Point Tests With High Pressure LOX at Elevated Temperatures, p. 4.
  9. ^ Arnold, Mark. 1U.S. Army Oxygen Generation System Development. RTO-MP-HFM-182. dtic.mil
  10. ^ Timmerhaus, K. D. (8 March 2013). Advances in Cryogenic Engineering: Proceedings of the 1957 Cryogenic Engineering Conference, National Bureau of Standards Boulder, Colorado, August 19–21, 1957. Springer Science & Business Media. pp. 150–. ISBN 978-1-4684-3105-6.
  11. ^ Belluscio, Alejandro G. (March 7, 2014). "SpaceX advances drive for Mars rocket via Raptor power". NASAspaceflight.com. Retrieved March 13, 2014.
  12. ^ Todd, David (November 20, 2012). "Musk goes for methane-burning reusable rockets as step to colonise Mars". FlightGlobal Hyperbola. Archived from the original on November 28, 2012. Retrieved November 22, 2012. 'We are going to do methane,' Musk announced as he described his future plans for reusable launch vehicles including those designed to take astronauts to Mars within 15 years, 'The energy cost of methane is the lowest and it has a slight Isp (Specific Impulse) advantage over Kerosene' said Musk adding, 'and it does not have the pain in the ass factor that hydrogen has.' ... SpaceX's initial plan will be to build a lox/methane rocket for a future upper stage codenamed Raptor. ... The new Raptor upper stage engine is likely to be only the first engine in a series of lox/methane engines.
  13. ^ "Oxygen - Density and Specific Weight". www.engineeringtoolbox.com. Retrieved 2021-05-19.
  14. ^ Cryogenics. Scienceclarified.com. Retrieved on 2012-07-22.