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 P: 128 Page 10 of this book desribe this. "In dealing with pahse transformations we are often concerned with the difference in free energy between two phases at temperatures away from the equilibrium temperature. For example, if a liquid is undercooled by a ΔT below T$_{m}$ beofre it solidifies, solidification will be accompanied by a decrease in free energy $\Delta$G. This free energy decrease provised the driving force for solidification" At temp T $\Delta$G = $\Delta$H -T$\Delta$S $\Delta$H = H$^{L}$ - H$^{S}$ $\Delta$S = S$^{L}$ - S$^{S}$ At eqilibrium melting temperature T$_{m}$ the free energies of the liquid and solid are the same. So $\Delta$G = 0 $\Delta$S = L/T$_{m}$ If the liquids is supercooled by even a small ammount then it become energically favourable for the liquid to become a solid and realese latent heat.