For a phase change H_2 O (l) (s)

$\Delta G^{{}^{\circ}}=-R T \ln K$ [ $\Delta G^{{}^{\circ}}=$ standard free energy change, $K=$ equilibrium constant] If a substance is in equilibrium between two phases at constant temperature and pressure, its chemical potential must have the same value in both the phases. \therefore $\Delta$G^{{}^{\circ}}=0 $\text{. }$