The entropy of fusion is the increase in entropy when melting a substance. This is almost always positive since the degree of disorder increases in the transition from an organized crystalline solid to the disorganized structure of a liquid; the only known exception is helium. It is denoted as ?Sfus and normally expressed in J mol−1 K−1
A natural process such as a phase transition will occur when the associated change in the Gibbs free energy is negative.
Since this is a thermodynamic equation, the symbol T refers to the absolute thermodynamic temperature, measured in kelvins (K).
Equilibrium occurs when the temperature is equal to the melting point
and the entropy of fusion is the heat of fusion divided by the melting point.
Helium
Helium-3 has a negative entropy of fusion at temperatures below 0.3 K. Helium-4 also has a very slightly negative entropy of fusion below 0.8 K. This means that, at appropriate constant pressures, these substances freeze with the addition of heat.