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There are two [[spin isomers of hydrogen]]; whereas room temperature hydrogen is mostly orthohydrogen, liquid hydrogen consists of 99.79% parahydrogen and 0.21% orthohydrogen.<ref name="IPTS-1968"/>
Hydrogen requires a theoretical minimum of 3.3 kWh/kg to liquefy, and 3.9 kWh/kg including converting the hydrogen to the para isomer, but practically generally takes 10-13 kWh/kg compared to a 33 kWh/kg heating value of hydrogen.<ref>https://www.hydrogen.energy.gov/pdfs/9013_energy_requirements_for_hydrogen_gas_compression.pdf {{bare URL PDF|date=August 2023}}</ref>
==History==
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