Liquid hydrogen

Liquid hydrogen
Names
	IUPAC name Hydrogen
	Systematic IUPAC name Liquid hydrogen
	Other names Hydrogen (cryogenic liquid), Refrigerated hydrogen; LH2, para-hydrogen
Identifiers
CAS Number	1333-74-0 ;
3D model (JSmol)	Interactive image;
ChEBI	CHEBI:33251 ;
ChemSpider	762 ;
KEGG	C00282 ;
PubChem CID	783;
RTECS number	MW8900000;
UNII	7YNJ3PO35Z ;
UN number	1966
	InChI InChI=1S/H2/h1H Key: UFHFLCQGNIYNRP-UHFFFAOYSA-N ; InChI=1/H2/h1H;
	SMILES [H][H];
Properties
Chemical formula	H2(l)
Molar mass	2.016 g·mol−1
Appearance	Colorless liquid
Density	0.07085 g/cm3 (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)
Hazards
	GHS labelling:
Pictograms
Signal word	Danger
Hazard statements	H220, H280
Precautionary statements	P210, P377, P381, P403
NFPA 704 (fire diamond)	3 4 0 CRYO
Autoignition; temperature	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). verify (what is ?) Infobox references

Liquid hydrogen (H₂(l)) is the liquid state of the element hydrogen. Hydrogen is found naturally in the molecular H₂ form.

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 for some time in thermally insulated containers.

There are two spin isomers of hydrogen: Room temperature hydrogen is 75% orthohydrogen. At cryogenic temperature it converts exothermically to parahydrogen. The thermodynamic lowest energy state for liquid hydrogen consists of 99.79% parahydrogen and 0.21% orthohydrogen. To avoid that the exothermic heat release occurs in storage, and thereby causes excessive boil-off, catalytic conversion to parahydrogen during liquification is employed.

Hydrogen requires a theoretical minimum of 3.3 kWh/kg (12 MJ/kg) to liquefy, and 3.9 kWh/kg (14 MJ/kg) including converting the hydrogen to the para isomer. Existing liquification facilities use 10–13 kWh/kg (36–47 MJ/kg) compared to a 33 kWh/kg (119 MJ/kg) heating value of hydrogen. More recent work shows future facilities are expected to cut the specific energy demand by half to 6.5 kWh/kg (23 MJ/kg)