The enthalpy of vaporization, (symbol ∆Hvap) also known as the (latent) heat of vaporization or heat of evaporation, is the energy (enthalpy) that must be added to the substance, typically a liquid, to transform a quantity of that substance into a gas. The enthalpy of vaporization is a function of the pressure at which that transformation takes place.
The enthalpy of vaporization is often quoted for the normal boiling temperature of the substance; although tabulated values are usually corrected to 298 K, that correction is often smaller than the uncertainty in the measured value.
The heat of vaporization is temperature-dependent, though a constant heat of vaporization can be assumed for small temperature ranges and for reduced temperature {\displaystyle T_{r}} T_r {\displaystyle \ll 1} \ll 1. The heat of vaporization diminishes with increasing temperature and it vanishes completely at a certain point called the critical temperature ( {\displaystyle T_{r}=1} {\displaystyle T_{r}=1}). Above the critical temperature, the liquid and vapor phases are indistinguishable, and the substance is called a supercritical fluid.
