# Entropy of fusion

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.[1] It is denoted as 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.

- , where is the enthalpy or heat of fusion.

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 so that

- ,

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.[2]

## See also

## Notes

- Atkins & Jones 2008, p. 236.
- Ott & Boerio-Goates 2000, pp. 92–93.

## References

- Atkins, Peter; Jones, Loretta (2008),
*Chemical Principles: The Quest for Insight*(4th ed.), W. H. Freeman and Company, p. 236, ISBN 0-7167-7355-4 - Ott, J. Bevan; Boerio-Goates, Juliana (2000),
*Chemical Thermodynamics: Advanced Applications*, Academic Press, ISBN 0-12-530985-6