Liquid hydrogen
Liquid hydrogen (H2(l)) is the liquid state of the element hydrogen. Hydrogen is found naturally in the molecular H2 form.[4]
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| Names | |||
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| IUPAC name
Hydrogen | |||
| Systematic IUPAC name
Liquid hydrogen | |||
| Other names
Hydrogen (cryogenic liquid), Refrigerated hydrogen; LH2, para-hydrogen | |||
| Identifiers | |||
3D model (JSmol) |
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PubChem CID |
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| RTECS number |
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| UNII | |||
| UN number | 1966 | ||
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| Properties | |||
| H2(l) | |||
| Molar mass | 2.016 g·mol−1 | ||
| Appearance | Colorless liquid | ||
| Density | 70.85 g/L (4.423 lb/cu ft)[1] | ||
| Melting point | −259.14 °C (−434.45 °F; 14.01 K)[2] | ||
| Boiling point | −252.87 °C (−423.17 °F; 20.28 K)[2] | ||
| Hazards | |||
| GHS labelling:[3] | |||
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| Danger | |||
| H220, H280 | |||
| P210, P377, P381, P403 | |||
| NFPA 704 (fire diamond) | |||
| 571 °C (1,060 °F; 844 K)[2] | |||
| Explosive limits | LEL 4.0%; UEL 74.2% (in air)[2] | ||
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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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).[5] 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.[6]
There are two spin isomers of hydrogen; liquid hydrogen consists of 99.79% parahydrogen and 0.21% orthohydrogen.[5]