What is Variance?

Example 4.10 (Three variances)

Limestone in a closed vessel, $\ce{CaCO3(s) <=> CaO(s) + CO2(g)}$: $n = 3$, $r = 1$, $\varphi = 3$ (two solids, one gas): $v = 1$. Choose $T$, and the pressure of carbon dioxide is fixed: $p = K^\circ(T)\,p^\circ$. A gas mixture of $\ce{N2}$, $\ce{H2}$ and $\ce{NH3}$ at equilibrium: $n = 3$, $r = 1$, $\varphi = 1$, $v = 3$ ($T$, $p$ and one composition variable); made from ammonia alone, or from a stoichiometric feed, $\ce{N2}$ and $\ce{H2}$ are in the ratio 1 : 3, $s = 1$ and $v = 2$: at given $T$ and $p$ the composition is fixed, as the weekend problem uses.