The standard enthalpy change of fusion, ΔHfus o, is the change in enthalpy when one mole of a substance in the liquid state is formed from the same substance in its solid state under standard conditions.
The process of melting is always endothermic, e.g.
Most data for the standard enthalpy change of fusion of substances are quoted at the melting points of those substances instead of at 298.15 K. The standard enthalpy change of solidification of a substance, ΔHsolid o, is the negative value of the standard enthalpy change of fusion of that substance.