Enthalpy of fusion is the change in enthalpy when a specified quantity of a substance changes from solid to liquid when heated. It is also referred to as latent heat of fusion.
Enthalpy of fusion is a latent heat because the substance's temperature is constant while melting, and the introduction of heat is not considered a temperature change. The solid state of a substance has a lower internal energy than its liquid state, so energy must be administered to melt a solid, and energy must be released to freeze a liquid. The enthalpy of fusion is a positive quantity with the exception of helium. At temperatures below 0.3 K, helium-3 has a negative enthalpy of fusion, and helium-4 is negative below 0.8 K. Helium-3 and helium-4 freeze when heat is added at certain constant pressures.