Liquid hydrogen

Liquid hydrogen

Names
IUPAC name Liquid hydrogen
Other names Hydrogen (cryogenic liquid); hydrogen, refrigerated liquid; LH2, para-hydrogen
Identifiers
CAS Number	1333-74-0 Y
3D model (Jmol)	Interactive image
ChEBI	CHEBI:33251 Y
ChemSpider	762 Y
KEGG	C00282 Y
PubChem	783
RTECS number	MW8900000
UNII	7YNJ3PO35Z Y
UN number	1966
InChI InChI=1S/H2/h1H Y Key: UFHFLCQGNIYNRP-UHFFFAOYSA-N Y InChI=1/H2/h1H
SMILES [H][H]
Properties
Chemical formula	H2
Molar mass	2.02 g·mol−1
Appearance	Colorless liquid
Density	70.85 g/L (4.423 lb/cu ft)
Melting point	−259.14 °C (14.01 K; −434.45 °F)
Boiling point	−252.87 °C (20.28 K; −423.17 °F)
Hazards
EU classification (DSD)
NFPA 704	4 3 0
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).
Y  (what is YN ?)
Infobox references

Liquid Hydrogen

RTECS	MW8900000
PEL-OSHA	Simple asphyxiant
ACGIH TLV-TWA	Simple asphyxiant

Liquid hydrogen (LH2 or LH₂) 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 hydrogen's critical point of 33 K. However, for hydrogen to be in a fully liquid state without boiling at atmospheric pressure, it needs to be cooled to 20.28 K (−423.17 °F/−252.87 °C). One 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. As in any gas, 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 pressurized and thermally insulated containers.

Liquid hydrogen consists of 99.79% parahydrogen, 0.21% orthohydrogen.

In 1885 Zygmunt Florenty Wróblewski published hydrogen's critical temperature as 33 K; critical pressure, 13.3 atmospheres; and boiling point, 23 K.

Hydrogen was liquefied by James Dewar in 1898 by using regenerative cooling and his invention, the vacuum flask. The first synthesis of the stable isomer form of liquid hydrogen, parahydrogen, was achieved by Paul Harteck and Karl Friedrich Bonhoeffer in 1929.

Room–temperature hydrogen consists mostly of the orthohydrogen form. After production, liquid hydrogen is in a metastable state and must be converted into the parahydrogen isomer form to avoid the exothermic reaction that occurs when it changes at low temperatures; this is usually performed using a catalyst like iron(III) oxide, activated carbon, platinized asbestos, rare earth metals, uranium compounds, chromium(III) oxide, or some nickel compounds.