Fugacity coefficient
= Fugacity coefficient
{title2=$\phi=f/P$}
The dimensionless ratio of a gas species' <chemical fugacity> to its pressure in the pure-fluid convention. Its <thermodynamic activity> relative to a gas reference pressure $P^\circ$ is $\phi P/P^\circ$. For mixtures the pressure denominator uses the species' partial pressure. The ideal-gas limit has $\phi=1$.