Orthohydrogen and Parahydrogen

An hydrogen molecule is made up of two atoms of hydrogen linked by a covalent bond. If we neglect the traces of deuterium and tritium which could be present, each hydrogen atom is constituted of a lone proton (this is called light hydrogen; the term protonium designates an atom of hydrogen where the electron has been replaced by an antiproton). This proton has an associated magnetic moment, which we can treat as being generated by the proton's spin. The molecule can thus be constituted either of two protons aligned in the same direction (this is orthohydrogen) or in opposite directions (this is parahydrogen).

Orthohydrogen is unstable at low temperatures, and spontaneously changes into parahydrogen with time, a process which releases heat; an undesirable occurrence when one wants hydrogen to remain liquid. At standard (room) temperature, hydrogen is made up of 75% orthohydrogen, a proportion which the liquefaction process preserves. One must therefore use a catalyst to accelerate the conversion of the liquid hydrogen into parahydrogen, or supply additional refrigeration equipment to absorb the heat that the liquid hydrogen will give off as it spontaneously converts itself to pure parahydrogen.