For a single component system, vapor (subscript $g$) and liquid (subscript $f$) coexist in mechanical, thermal and phase equilibrium when
- $u_{g}=u_{f}$ (equality of specific internal energy)
- $h_{g}=h_{f}$ (equality of specific enthalpy)
- $s_{g}=s_{f}$ (equality of specific entropy)
- $g_{g}=g_{f}$ (equality of specific Gibbs free energy)