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Liquid hydrogen 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). 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 for 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.

Wikipedia, Liquid hydrogen