The ortho-to-para ratio in H2 is an important consideration in the liquefaction and storage of liquid hydrogen: the conversion from ortho to para is Hydrogen is a chemical element; it has the symbol H and atomic number 1. It is the lightest and most abundant chemical element in the universe, constituting about 75% of all normal matter. Under standard conditions, hydrogen is a gas of diatomic molecules with the formula H2, called dihydrogen, or sometimes hydrogen gas, molecular hydrogen, or simply hydrogen. Dihydrogen is colorless, odorless, no Molecular H2 exists as two nuclear isomers that differ in the spin states of their nuclei. In the ortho­hydrogen form, the spins of the two nuclei are parallel, forming a spin triplet state having a total molecular spin S = 1 {\displaystyle S=1} ; in the para­hydrogen form the spins are anti­parallel and form a spin singlet state having spin S = 0 {\displaystyle S=0} . The equilibrium ratio of ortho- to para-hydrogen depends on temperature. At room temperature or warmer, equilibrium hydrogen gas contains about 25% of the para form and 75% of the ortho form. The ortho form is an excited state, having higher energy than the para form by 1.455 kJ/mol, and it converts to the para form over the course of several minutes when cooled to low temperature. The thermal properties of these isomers differ because each has distinct rotational quantum states. The ortho-to-para ratio in H2 is an important consideration in the liquefaction and storage of liquid hydrogen: the conversion from ortho to para is exothermic, and produces sufficient heat to evaporate most of the liquid if the conversion to para­hydrogen does not occur during the cooling process. Catalysts for the ortho-para inter­conversion, such as ferric oxide and activated carbon compounds, are therefore used during hydrogen cooling to avoid this loss of liquid.

Depending upon the direction of the spin of the nuclei, the hydrogens are of two types: Ortho hydrogen molecules are those in which the spins of both the nuclei are in the same direction. Molecules of hydrogen in which the spins of both the nuclei are in the opposite direction are called para hydrogen. Ordinary dihydrogen is an equilibrium mixture of ortho and para hydrogen. \[\text{ortho hydrogen} \ce{<=>} \text{para hydrogen} \nonumber\] The amount of ortho and para hydrogen varies with temperature as: - At 0°K, hydrogen contains mainly para hydrogen which is more stable. - At the temperature of liquefaction of air, the ratio of ortho and para hydrogen is 1 : 1. - At the room temperature, the ratio of ortho to para hydrogen is 3 : 1. - Even at very high temperatures, the ratio of ortho to para hydrogen can never be more than 3 : 1. Thus, it has been possible to get pure para hydrogen by cooling ordinary hydrogen gas to a very low temperature (close to 20 K) but it is never possible to get a sample of hydrogen containing more than 75% of ortho hydrogen.