Liquid hydrogen (H2(l)) is the liquid state of the element hydrogen. Hydrogen is found naturally in the molecular H2 form.
|Systematic IUPAC name
Hydrogen (cryogenic liquid), Refrigerated hydrogen; LH2, para-hydrogen
3D model (JSmol)
|Molar mass||2.016 g·mol−1|
|Density||70.85 g/L (4.423 lb/cu ft)|
|Melting point||−259.14 °C (−434.45 °F; 14.01 K)|
|Boiling point||−252.87 °C (−423.17 °F; 20.28 K)|
|P210, P377, P381, P403|
|NFPA 704 (fire diamond)|
|571 °C (1,060 °F; 844 K)|
|Explosive limits||LEL 4.0%; UEL 74.2% (in air)|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
To exist as a liquid, H2 must be cooled below its critical point of 33 K. However, for it to be in a fully liquid state at atmospheric pressure, H2 needs to be cooled to 20.28 K (−252.87 °C; −423.17 °F). 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.
There are two spin isomers of hydrogen; liquid hydrogen consists of 99.79% parahydrogen and 0.21% orthohydrogen.