Frozen ortho-para mixture ($C_{V,m}=x_o{C}_{V,m,o}+x_p{C}_{V,m,p}$)

A mixture whose orthohydrogen and parahydrogen mole fractions are fixed on the observation timescale, although each species can equilibrate over its own rotational levels. Its molar heat capacity is $x_o{C}_o+x_p{C}_p$. normal hydrogen retained during cooling has $x_o=3/4$ and $x_p=1/4$. A mixture initially prepared at some other equilibrium composition can also be frozen; its subsequent heat response does not include a changing-composition term.