= Frozen ortho-para mixture
{title2=$C_{V,m}=x_oC_{V,m,o}+x_pC_{V,m,p}$}
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 $x_oC_o+x_pC_p$. <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.
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