# GATE2019-4

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