Editing Liquid hydrogen (section)

==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>