During a phase change such as vaporization or condensation, a substance can absorb or release energy while temperature and pressure remain fixed at their saturation values.

For vaporization at fixed pressure, the specific enthalpy change between saturated liquid and saturated vapor is

$$h_{fg}=h_g-h_f.$$

This is the latent enthalpy of vaporization. Condensation releases the same magnitude in the reverse direction.

For a mass $m$ undergoing complete vaporization at the saturation pressure,

$$Q\approx m h_{fg}$$

when kinetic, potential and other work effects are negligible.

If $1.5\ \mathrm{kg}$ of a saturated liquid has $h_{fg}=2200\ \mathrm{kJ/kg}$, complete vaporization requires approximately

$$Q=1.5(2200)=3300\ \mathrm{kJ}.$$

Latent heat is distinct from sensible heating. Sensible heating changes temperature within a phase; latent energy changes the fraction of matter in each phase. Vapor power and refrigeration cycles exploit this large enthalpy change.