How are ortho-hydrogen and para-hydrogen different?

Ortho-hydrogen and para-hydrogen are two distinct modifications of molecular hydrogen that differ based on the spin alignment of their two protons.1

• Ortho-hydrogen: The spins of the two protons are aligned in the same direction (parallel).12
• Para-hydrogen: The spins of the two protons are aligned in opposite directions (antiparallel), resulting in a total nuclear angular momentum of zero.13

These spin alignments lead to differences in magnetic properties and slight variations in physical properties, such as melting point and vapour pressure. Because of these differences, the two forms can be separated using low-temperature gas chromatography.1

While they are normally distinct, they can interconvert under specific conditions, such as through the use of catalysts like activated charcoal, electrical discharge, or high temperatures. Ordinary hydrogen gas typically exists as an equilibrium mixture of three parts ortho-hydrogen to one part para-hydrogen.12