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'''Liquid hydrogen''' (H<sub>2(l)</sub>) 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 [[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 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>
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