= Solution
For solar elemental abundances, take a <molecular hydrogen>- and <helium>-dominated atmosphere with <atmospheric carbon-to-oxygen ratio> near $0.55$. The principal oxygen-bearing molecule is generally <water>; the carbon and nitrogen carriers depend on both <temperature> and <pressure>.
At the cool $500\,\mathrm{K}$ observable upper layers, <methane> is the expected main carbon reservoir under <chemical equilibrium>, with abundant <water> and much less <carbon monoxide>. At the hotter layers approaching $1400\,\mathrm{K}$, <carbon monoxide> becomes the main carbon carrier and <water> contains much of the <oxygen> not bound in it. The relevant <law of mass action> follows from
$$
\mathrm{CO}+3\mathrm{H}_2\rightleftharpoons\mathrm{CH}_4+\mathrm{H}_2\mathrm{O},\qquad
K_p(T)=\frac{(p_{\rm CH_4}/p^\circ)(p_{\rm H_2O}/p^\circ)}{(p_{\rm CO}/p^\circ)(p_{\rm H_2}/p^\circ)^3}.
$$
Lower <temperature> favors the exothermic methane-forming direction, while increasing <pressure> favors the side with fewer molecules. This explains why the change of dominant carrier cannot be specified by <temperature> alone.
Nitrogen is distributed between <molecular nitrogen> and <ammonia>. Cooling favors <ammonia>, but low <pressure> favors <molecular nitrogen>; it is therefore unsafe to call <ammonia> dominant throughout the $500\,\mathrm{K}$ low-pressure region. The exact partition requires the <equilibrium constant> for $\mathrm{N}_2+3\mathrm{H}_2\rightleftharpoons2\mathrm{NH}_3$.
<Carbon dioxide> is usually a minor constituent at solar composition; <hydrogen cyanide> and <acetylene> are much less abundant than the principal carbon carriers in this oxygen-rich equilibrium case. Condensation can remove refractory species where a condensation curve is crossed. \b[The robust cool-atmosphere expectation is a <molecular hydrogen>–<helium> background with <methane> and <water>, changing toward <carbon monoxide> in hotter layers.] Precise mixing ratios require thermodynamic data and an explicitly specified elemental inventory after rainout.
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