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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}}K.▼
[[File:Liquid_hydrogen_bubblechamber.jpg|thumb|Liquid hydrogen bubbles forming in two glass flasks at the [[Bevatron]] laboratory
[[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, c. 1950s
▲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]].
==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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