The heat required during a change of state depends upon the heat of transformation and the mass of the substance undergoing a change of state. Thus, if mass *m *of a substance undergoes a change from one state to the other, then the quantity of heat required is given by

where L is known as latent heat and is a characteristic of the substance.

Its SI unit is J kg^{-1}. The value of L also depends on the pressure.

## Latent heat of Fusion:

The latent heat of fusion (L_{f}) is the heat per unit mass required to change a substance from solid into liquid at the same temperature and pressure.

L_{f }= 3.3 × 10^{5} J kg^{-1}

## Latent heat of Vaporization:

The latent heat of vaporization (L_{V}) is the heat per unit mass required to change a substance from liquid to the vapor state without the change in the temperature and pressure.

L_{V} = 22.6 × 10^{5 }J