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| ImageFileL1 = Dihydrogen-2D-dimensions.png
| ImageFileR1 = Dihydrogen-3D-vdW.png
| ImageFile2 =
| verifiedrevid = 476996812
| IUPACName = Hydrogen
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}}
|Section2={{Chembox Properties
| Formula
| H=2
| Appearance = Colorless liquid
| Density = {{convert|
| 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
| BoilingPtC = −252.87
| BoilingPt_ref = <ref name="h"/>}}
|Section7={{Chembox Hazards
| ExternalSDS =
| GHSPictograms = {{GHS02}}{{GHS04}}
| GHSSignalWord = danger
| HPhrases
| PPhrases
| GHS_ref
| MainHazards =
| NFPA-H = 3
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| AutoignitionPt_ref = <ref name="h"/>
| ExploLimits = LEL 4.0%; UEL 74.2% (in air)<ref name="h"/>
| PEL =
}}
'''Liquid hydrogen''' (
To exist as a liquid, H<sub>2</sub> must be cooled below its [[critical point (thermodynamics)|critical point]] of 33 [[Kelvins|K]]. However, for it to be in a fully liquid state at [[atmospheric pressure]], H<sub>2</sub> needs to be cooled to {{convert|20.28|K|C F}}.<ref name="IPTS-1968">[http://media.iupac.org/publications/pac/1970/pdf/2203x0555.pdf IPTS-1968], iupac.org, accessed 2020-01-01</ref> A common method of obtaining liquid hydrogen involves a [[compressor]] resembling a jet engine in both appearance and principle. Liquid hydrogen is typically used as a concentrated form of [[hydrogen storage]]. Storing it as liquid takes less space than storing it as a gas at normal temperature and pressure. However, the liquid density is very low compared to other common fuels. Once liquefied, it can be maintained as a liquid for some time in thermally insulated containers.<ref>{{Cite web |title=Liquid Hydrogen Delivery |url=https://www.energy.gov/eere/fuelcells/liquid-hydrogen-delivery |access-date=2022-07-30 |website=Energy.gov |language=en}}</ref>
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==
{{further|Timeline of low-temperature technology}}
[[File:Air Products Headquarters, Trexlertown.JPG|thumb|left|The global headquarters of [[Air Products]] in [[Trexlertown, Pennsylvania]], a leading global supplier of liquid hydrogen]]
In 1885, [[Zygmunt Florenty Wróblewski]] published hydrogen's critical temperature as {{convert|33|K|C F}}; critical pressure, 13.3 atmospheres; and boiling point, {{convert|23|K|C F}}.▼
[[File:Liquid_hydrogen_bubblechamber.jpg|thumb|Liquid hydrogen bubbles forming in two glass flasks at the [[Bevatron]] laboratory in 1955]]▼
[[File:Hydrogen Tank - GPN-2000-001458.jpg|thumb|A large hydrogen tank in a vacuum chamber at
[[Image:Linde-Wasserstofftank.JPG|
[[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
[[Hydrogen]] was liquefied by [[James Dewar]] in 1898 by using [[regenerative cooling]] and his invention, the [[vacuum flask]]. The first synthesis of the stable isomer form of liquid hydrogen, parahydrogen, was achieved by [[Paul Harteck]] and [[Karl Friedrich Bonhoeffer]] in 1929.
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==Uses==
Liquid hydrogen is a common [[liquid fuel|liquid]] [[rocket propellant|rocket fuel]] for [[spacecraft propulsion|rocketry]]
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.
Liquid hydrogen can be used as the fuel for an [[internal combustion engine]] or [[fuel cell]]. Various submarines, including the
▲Liquid hydrogen can be used as the fuel for an [[internal combustion engine]] or [[fuel cell]]. Various submarines ([[Type 212 submarine]], [[Type 214 submarine]]) and concept [[hydrogen vehicle]]s have been built using this form of hydrogen (see [[DeepC]], [[BMW H2R]]). Due to its similarity, builders can sometimes modify and share equipment with systems designed for [[liquefied natural gas]] (LNG). Liquid hydrogen is being investigated as a [[Zero-carbon|zero carbon fuel]] for [[Hydrogen-powered aircraft|aircraft]]. However, because of the lower [[Energy density#Energy density in energy storage and in fuel|volumetric energy]], the hydrogen volumes needed for combustion are large. Unless [[fuel injection#Direct injection systems|direct injection]] is used, a severe gas-displacement effect also hampers maximum breathing and increases pumping losses.
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
The density of liquid hydrogen is only 70.85 g/L (at 20 [[kelvin|K]]), a [[relative density]] of just 0.07. Although the specific energy is more than twice that of other fuels, this gives it a remarkably low volumetric [[energy density]], many fold lower.▼
▲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.▼
▲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]].
The [[triple point]] of hydrogen is at 13.81 K<ref name="IPTS-1968"/> 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>▼
▲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>
▲File:Liquid_hydrogen_bubblechamber.jpg|Liquid hydrogen bubbles forming in two glass flasks at the [[Bevatron]] laboratory in 1955
▲File:Hydrogen Tank - GPN-2000-001458.jpg|A large hydrogen tank in a vacuum chamber at Lewis Research Center in 1967
▲Image:Linde-Wasserstofftank.JPG|Tank for liquid hydrogen of [[Linde AG|Linde]], [[Museum Autovision]], [[Altlußheim]], [[Germany]]
==Safety==
Due to its cold temperatures, liquid hydrogen is a hazard for [[Frostbite|cold burns]]. Hydrogen itself is biologically inert and its only human health hazard as a vapor is displacement of oxygen, resulting in asphyxiation, and its very high flammability and ability to detonate when mixed with air. Because of its flammability, liquid hydrogen should be kept away from heat or flame unless ignition is intended. Unlike ambient
==See also==
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*[[Liquid hydrogen tank car]]
*[[Liquid hydrogen tanktainer]]
*[[Hydrogen tanker]]
{{div col end}}
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{{reflist|30em}}
[[Category:Hydrogen physics]]
[[Category:Hydrogen technologies]]
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