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| H=2
| Appearance = Colorless liquid
| Density = {{convert|0.07085|g/cm3|lb/ft3|abbr=on}}<ref>[http://webbook.nist.gov/cgi/fluid.cgi?Action=Load&ID=C1333740&Type=SatT&Digits=5&PLow=.5&PHigh=1.5&PInc=.1&RefState=DEF&TUnit=K&PUnit=atm&DUnit=kg/m3&HUnit=kJ/mol&WUnit=m/s&VisUnit=uPa*s&STUnit=N/m Thermophysical Properties of Hydrogen]
| MeltingPtC = −259.14
| MeltingPt_ref = <ref name="h">[http://www.safety.seas.harvard.edu/services/hydrogen.html ''Information specific to liquid hydrogen''] {{webarchive|url=https://web.archive.org/web/20090717083849/http://www.safety.seas.harvard.edu/services/hydrogen.html |date=2009-07-17}}, harvard.edu, accessed 2009-06-12</ref>
| BoilingPtC = −252.87
| BoilingPt_ref = <ref name="h"/>}}
|Section7={{Chembox Hazards
| ExternalSDS =
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| AutoignitionPt_ref = <ref name="h"/>
| ExploLimits = LEL 4.0%; UEL 74.2% (in air)<ref name="h"/>
| PEL =
}}
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There are two [[spin isomers of hydrogen]]; whereas room temperature hydrogen is mostly orthohydrogen, liquid hydrogen consists of 99.79% parahydrogen and 0.21% orthohydrogen.<ref name="IPTS-1968"/>
Hydrogen requires a theoretical minimum of {{convert|3.3
==History==
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[[File:Hydrogen Tank - GPN-2000-001458.jpg|thumb|A large hydrogen tank in a vacuum chamber at the [[Glenn Research Center]] in [[Brook Park, Ohio]], in 1967]]
[[Image:Linde-Wasserstofftank.JPG|thumb|A [[Linde AG]] tank for liquid hydrogen at the [[Museum Autovision]] in [[Altlußheim]], Germany, in 2008]]
[[File:DOT Hazardous Material Placard liquid hydrogen.jpg|thumb|Two [[United States Department of Transportation|U.S. Department of Transportation]] placards indicating the presence of [[hazardous materials]], which are used with liquid hydrogen]]
In 1885, [[Zygmunt Florenty Wróblewski]] published hydrogen's critical temperature as {{convert|33|K|C F}}; critical pressure, {{convert|13.3|atm|psi}}; and boiling point, {{convert|23|K|C F}}.
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==Uses==
Liquid hydrogen is a common [[liquid fuel|liquid]] [[rocket propellant|rocket fuel]] for [[spacecraft propulsion|rocketry]] application and is used by [[NASA]] and the [[United States Air Force|U.S. Air Force]], which operate a large number of liquid hydrogen tanks with an individual capacity up to 3.8 million liters (1 million U.S. gallons).<ref name="Flynn2004">{{cite book|author=Flynn, Thomas |title=Cryogenic Engineering, Second Edition, Revised and Expanded|url=https://books.google.com/books?id=-XfMBQAAQBAJ&pg=PA401|date=2004|publisher=CRC Press|isbn=978-0-203-02699-1|page=401}}</ref>
In most [[rocket engine]]s fueled by liquid hydrogen, it first [[regenerative cooling (rocket)|cools]] the nozzle and other parts before being mixed with the oxidizer, usually [[liquid oxygen]], and burned to produce water with traces of [[ozone]] and [[hydrogen peroxide]]. Practical H<sub>2</sub>–O<sub>2</sub> rocket engines run fuel-rich so that the exhaust contains some unburned hydrogen. This reduces combustion chamber and nozzle erosion. It also reduces the molecular weight of the exhaust, which can increase [[specific impulse]], despite the incomplete combustion.
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Liquid hydrogen is also used to cool neutrons to be used in [[neutron scattering]]. Since neutrons and hydrogen nuclei have similar masses, kinetic energy exchange per interaction is maximum ([[elastic collision]]). Finally, superheated liquid hydrogen was used in many [[bubble chamber]] experiments.
The first [[Thermonuclear weapon|thermonuclear bomb]], [[Ivy Mike]], used liquid [[deuterium]], also known as
==Properties==
The product of hydrogen combustion in a pure oxygen environment is solely water vapor. However, the high combustion temperatures and present atmospheric nitrogen can result in the breaking of N≡N bonds, forming toxic NOx if no exhaust scrubbing is done.<ref>{{Cite journal |last=Lewis |first=Alastair C. |date=2021-07-22 |title=Optimising air quality co-benefits in a hydrogen economy: a case for hydrogen-specific standards for NOx emissions |journal=Environmental Science: Atmospheres |language=en |volume=1 |issue=5 |pages=201–207 |doi=10.1039/D1EA00037C |s2cid=236732702 |issn=2634-3606|doi-access=free}}</ref> Since water is often considered harmless to the environment, an engine burning it can be considered "zero emissions". In aviation, however, water vapor emitted in the atmosphere contributes to [[global warming]] (to a lesser extent than CO<sub>2</sub>).<ref>{{cite journal |last1=Nojoumi |first1=H. |title=Greenhouse gas emissions assessment of hydrogen and kerosene-fueled aircraft propulsion |journal=International Journal of Hydrogen Energy |date=2008-11-10 |volume=34 |issue=3 |pages=1363–1369 |doi=10.1016/j.ijhydene.2008.11.017}}</ref> Liquid hydrogen also has a much higher [[specific energy]] than gasoline, natural gas, or diesel.<ref name="almc.army.mil">[http://www.almc.army.mil/alog/issues/MayJun00/MS492.htm Hydrogen As an Alternative Fuel] {{webarchive|url=https://web.archive.org/web/20080808053811/http://www.almc.army.mil/alog/issues/MayJun00/MS492.htm |date=2008-08-08}}. Almc.army.mil. Retrieved on 2011-08-28.</ref>
The density of liquid hydrogen is only 70.85
Liquid hydrogen requires [[cryogenic]] storage technology such as special thermally insulated containers and requires special handling common to all [[cryogenic fuel]]s. This is similar to, but more severe than [[liquid oxygen]]. Even with thermally insulated containers it is difficult to keep such a low temperature, and the hydrogen will gradually leak away (typically at a rate of 1% per day<ref name="almc.army.mil"/>). It also shares many of the same [[hydrogen safety|safety issues]] as other forms of hydrogen, as well as being cold enough to liquefy, or even solidify atmospheric oxygen, which can be an explosion hazard.
The [[triple point]] of hydrogen is at 13.81 K<ref name="IPTS-1968"/> and 7.042 kPa.<ref>Cengel, Yunus A. and Turner, Robert H. (2004). ''Fundamentals of thermal-fluid sciences'', McGraw-Hill, p. 78, {{ISBN|0-07-297675-6}}</ref>
==Safety==
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{{reflist|30em}}
[[Category:Hydrogen physics]]
[[Category:Hydrogen technologies]]
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