Latent heat of fusion
The heat needed to melt a unit mass of a solid at its melting point — 334 kJ/kg for water — released again on freezing.
Definition
The latent heat of fusion is the energy required to melt one unit mass of a solid at its melting point without raising its temperature, Q = m L_f. For water it is 334 kJ/kg — comparable to the energy needed to heat the same mass of liquid water from 0 °C to about 80 °C, which is why ice is such an effective thermal buffer.
The energy goes into loosening the rigid bonds of the crystal lattice so the molecules can flow as a liquid. The identical quantity is released when the liquid refreezes, which is why spraying crops with water can protect them from frost: the freezing water holds the buds at 0 °C as it gives up its heat of fusion.
On a heating curve the heat of fusion appears as the first flat plateau, where temperature holds at the melting point until the last of the solid has melted.
History
Quantified through the calorimetry founded by Joseph Black, whose study of melting ice introduced the concept of latent heat.