= Relative volatility
{title2=$\alpha=\dfrac{y/(1-y)}{x/(1-x)}$}
{wiki}
For a two-component <mixture>, let $x$ be component A's <mole fraction> in the <liquid> and $y$ its <mole fraction> in the coexisting <vapor>. At interior compositions, define the relative volatility of A with respect to B by the ratio of composition odds:
$$
\alpha=\frac{y/(1-y)}{x/(1-x)}.
$$
Thus $\alpha>1$ means A is enriched in the <vapor>, while $\alpha=1$ means $y=x$. Solving this definition gives
$$
\alpha x(1-y)=y(1-x),\qquad
y=\frac{\alpha x}{1+(\alpha-1)x}.
$$
Here a constant <relative volatility> is a stipulated model of <vapor-liquid equilibrium>, not a measured property of a named <chemical solution>. Real <relative volatility> can vary with composition, <temperature>, and <pressure>. The relation specifies no absolute <temperature> scale: even fixing $\alpha=2$ does not determine a <boiling>-<temperature> curve or a <heat> requirement.
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