The amount of heat required to change a solid of unit mass to its liquid state at a constant temperature is the **latent heat of fusion** of that material.

Similarly, the amount of heat that needs to be extracted to change a liquid of unit mass into its solid state at a constant temperature is called the **latent heat of solidificatio**n of that liquid.

The latent heat of fusion and the latent heat of solidification of a substance are equal. Latent heat is generally denoted by L.

If H is the quantity of heat that is to be supplied to or extracted from a mass m of a substance to melt or solidify it then H = mL.

Unit of latent heat = unit of heat/unit of mass

The unit of latent heat in SI is J.kg^{-1}