The energy to change one mole of a substance from a liquid to a vapor at its boiling point is

Its enthalpy of vaporization.

Explanation:

It takes energy to overcome the intermolecular forces of attraction between the molecules in a liquid and force them into the vapour phase.

The process of moving molecules from the liquid phase into the vapour state is vaporization, so the amount of heat required is the enthalpy of vaporization (also called the heat of vaporization).

law of conservation of matter