 Some organic compounds melt through mesophases, states of partial order between solid and liquid.From a thermodynamics point of view, at the melting point the change in Gibbs free energy "∆G" of the substances is zero, but there are non-zero changes in the enthalpy ("H") and the entropy ("S"), known respectively as the enthalpy of fusion (or latent heat of fusion) and the entropy of fusion.Melting is therefore classified as a first-order phase transition.