Thermodynamics of Melting
When a substance melts and the solid and liquid phases are in an equilibrium, it maintains a constant temperature, the melting point. The energy used for melting is a latent heat. This characterizes the process of melting as a first-order phase transition.
From a thermodynamics point of view, at the melting point the change in Gibbs free energy of the material is zero, but the enthalpy and the entropy of the material are increasing . Melting occurs when the Gibbs free energy of the liquid becomes lower than the solid for that material. The temperature at which this occurs is dependent on the ambient pressure.
Low-temperature helium is the only known exception to the general rule. Helium-3 has a negative enthalpy of fusion at temperatures below 0.3 K. Helium-4 also has a very slightly negative enthalpy of fusion below 0.8 K. This means that, at appropriate constant pressures, heat must be removed from these substances in order to melt them.
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