Q.Which one of the following statements is incorrect with regard to ortho and para dihydrogen?

The single proton that makes up a hydrogen nucleus carries a nuclear spin. When two hydrogen atoms bond to form H₂, the two nuclear spins can point the same way (parallel) or opposite ways (antiparallel), and because these two arrangements are physically distinguishable, the H₂ molecule effectively exists as two different nuclear-spin isomers:

• Ortho-hydrogen -- nuclear spins parallel.
• Para-hydrogen -- nuclear spins antiparallel (opposed).

At room temperature, ordinary hydrogen gas is a roughly 75:25 mixture of ortho- to para-hydrogen. The ortho form is the more thermodynamically stable of the two at ordinary temperature, so interconversion between the isomers happens only slowly on its own; but as temperature falls, the equilibrium shifts to favour the para form, so at very low temperatures almost all the gas becomes para-hydrogen. Converting stored para-hydrogen back to the ortho form (needed, since pure para-hydrogen slowly reverts and releases heat that can boil off a cryogenic storage vessel) can be sped up catalytically with platinum or iron, by an electric discharge, by heating above 800°C, or by mixing with paramagnetic species (O₂, NO, NO₂) or with atomic/nascent hydrogen.

Because nuclear spin does not influence electron bonding, the two isomers are chemically identical; the small differences that do exist are physical. Para-hydrogen's melting point (13.83 K) and boiling point (20.26 K) are both a shade lower than ortho-hydrogen's (13.95 K and 20.39 K respectively). And because para-hydrogen's two opposed nuclear spins cancel, it has zero net nuclear magnetic moment, while ortho-hydrogen's two parallel spins add, giving it a magnetic moment twice that of a single proton.