Editing Liquid hydrogen

{{short description|Liquid state of the element hydrogen}}
{{chembox
| Watchedfields = changed
| ImageFileL1 = Dihydrogen-2D-dimensions.png
| ImageFileR1 = Dihydrogen-3D-vdW.png
| ImageFile2 = File:Liquid_hydrogen_bubblechamber.jpg
| verifiedrevid = 476996812
| IUPACName = Hydrogen
| SystematicName = Liquid hydrogen
| OtherNames = Hydrogen (cryogenic liquid), Refrigerated hydrogen; LH{{sub|2}}, para-hydrogen
|Section1={{Chembox Identifiers
| Abbreviations = 
| StdInChI_Ref = {{stdinchicite|correct|chemspider}}
| StdInChI = 1S/H2/h1H
| StdInChIKey_Ref = {{stdinchicite|correct|chemspider}}
| StdInChIKey = UFHFLCQGNIYNRP-UHFFFAOYSA-N
| CASNo = 1333-74-0
| CASNo_Ref = {{cascite|correct|CAS}}
| ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}}
| ChemSpiderID = 762
| UNNumber = [[List of UN Numbers 1901 to 2000|1966]]
| EINECS = 
| PubChem = 783
| SMILES = [H][H]
| UNII_Ref = {{fdacite|correct|FDA}}
| UNII = 7YNJ3PO35Z
| InChI = 1/H2/h1H
| RTECS = MW8900000
| MeSHName =
| ChEMBL_Ref = {{ebicite|correct|EBI}}
| ChEBI_Ref = {{ebicite|correct|EBI}}
| ChEBI = 33251
| KEGG_Ref = {{keggcite|correct|kegg}}
| KEGG = C00282
}}
|Section2={{Chembox Properties
| Formula = {{chem2|H2([[Liquid|l]])}}
| 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], nist.gov, accessed 2012-09-14</ref> 
| 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 =
| GHSPictograms = {{GHS02}}{{GHS04}}
| GHSSignalWord = danger
| HPhrases = {{HPhrases|H220|H280}}
| PPhrases = {{PPhrases|P210|P377|P381|P403}}
| GHS_ref = <ref>GHS: [https://gestis.dguv.de/data?name=007010 GESTIS 007010]</ref>
| MainHazards = 
| NFPA-H = 3
| NFPA-F = 4
| NFPA-R = 0
| NFPA-S = CRYO
| FlashPt =
| AutoignitionPtC = 571
| AutoignitionPt_ref = <ref name="h"/> 
| ExploLimits = LEL 4.0%; UEL 74.2% (in air)<ref name="h"/>
| PEL =}}
}}

'''Liquid hydrogen''' ({{chem2|H2(l)}}) is the [[liquid state]] of the element [[hydrogen]]. Hydrogen is found naturally in the [[molecule|molecular]] H<sub>2</sub> form.<ref>{{cite web |title=We've Got (Rocket) Chemistry, Part 1 |url=https://blogs.nasa.gov/Rocketology/tag/liquid-hydrogen/ |website=NASA Blog |date=15 April 2016 |access-date=3 October 2021}}</ref><!-- this is true in Earth's atmosphere, but not generically -->

To exist as a liquid, H<sub>2</sub> must be cooled below its [[critical point (thermodynamics)|critical point]] of 33&nbsp;[[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|kWh/kg|MJ/kg|abbr=on}} to liquefy, and {{convert|3.9|kWh/kg|MJ/kg|abbr=on}} including converting the hydrogen to the para isomer, but practically generally takes {{convert|10-13|kWh/kg|MJ/kg|abbr=on}} compared to a {{convert|33|kWh/kg|MJ/kg|0|abbr=on}} heating value of hydrogen.<ref>{{Cite report |url=https://www.hydrogen.energy.gov/pdfs/9013_energy_requirements_for_hydrogen_gas_compression.pdf |title=DOE Hydrogen and Fuel Cells Program Record: Energy requirements for hydrogen gas compression and liquefaction as related to vehicle storage needs |last=Gardiner |first=Monterey |date=2009-10-26 |publisher=United States Department of Energy |issue=9013}}</ref>

==History==
{{further|Timeline of low-temperature technology}}
[[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 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}}.

[[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.

==Spin isomers of hydrogen==
{{main|Spin isomers of hydrogen}}
The two nuclei in a dihydrogen molecule can have two different [[Spin (physics)|spin]] states.
Parahydrogen, in which the two [[nuclear spin]]s are antiparallel, is more stable than orthohydrogen, in which the two are parallel. At room temperature, gaseous hydrogen is mostly in the ortho isomeric form due to thermal energy, but an ortho-enriched mixture is only [[metastability|metastable]] when liquified at low temperature. It slowly undergoes an [[exothermic reaction]] to become the para isomer, with enough energy released as heat to cause some of the liquid to boil.<ref name="UFLnotes"/> To prevent loss of the liquid during long-term storage, it is therefore intentionally converted to the para isomer as part of the production process, typically using a [[catalyst]] such as [[iron(III) oxide]], [[activated carbon]], platinized asbestos, rare earth metals, uranium compounds, [[chromium(III) oxide]], or some nickel compounds.<ref name="UFLnotes">{{cite web |url= http://www.phys.ufl.edu/courses/phy4550-6555c/spring11/liquefaction-2011.pdf |title= Liquefaction of "Permanent" Gases |date= 2011 |access-date= 2017-10-16 |format= PDF of lecture notes}}</ref>

==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.

Liquid hydrogen can be used as the fuel for an [[internal combustion engine]] or [[fuel cell]]. Various submarines, including the [[Type 212 submarine]], [[Type 214 submarine]], and others, and concept [[hydrogen vehicle]]s have been built using this form of hydrogen, such as the [[DeepC]], [[BMW H2R]], and others. 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]]. 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 hydrogen-2, for nuclear fusion.

==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|bibcode=2021ESAt....1..201L }}</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&nbsp;kg/m<sup>3</sup> (at 20&nbsp;[[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.

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&nbsp;K<ref name="IPTS-1968"/> and 7.042&nbsp;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==
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-temperature gaseous hydrogen, which is lighter than air, hydrogen recently vaporized from liquid is so cold that it is heavier than air and can form flammable heavier-than-air air–hydrogen mixtures.

==See also==
{{Div col|colwidth=25em}}
*[[Industrial gas]]
*[[Liquefaction of gases]]
*[[Hydrogen safety]]
*[[Compressed hydrogen]]
*[[Cryo-adsorption]]
*[[Expansion ratio]]
*[[Gasoline gallon equivalent]]
*[[Slush hydrogen]]
*[[Solid hydrogen]]
*[[Metallic hydrogen]]
*[[Hydrogen infrastructure]]
*[[Hydrogen-powered aircraft]]
*[[Liquid hydrogen tank car]]
*[[Liquid hydrogen tanktainer]]
*[[Hydrogen tanker]]
{{div col end}}

==References==
{{reflist|30em}}

[[Category:Hydrogen physics]]
[[Category:Hydrogen technologies]]
[[Category:Hydrogen storage]]
[[Category:Liquid fuels]]
[[Category:Rocket fuels]]
[[Category:Coolants]]
[[Category:Cryogenics]]
[[Category:Hydrogen]]
[[Category:Industrial gases]]
[[Category:1898 in science]]