The heat of vaporization formula is q = m x ?Hv, where "q" is heat energy, "m" is mass and "?Hv" is heat of vaporization. This represents the energy needed to heat a liquid into a gaseous state.
For example, the heat energy needed to turn 25 grams of water into steam (gaseous state) is: 25 grams x 2257 Joules/gram or 56425 Joules. The 2257 J/g is a predetermined value for heat of vaporization of water.
A practical application of the heat of vaporization is when a drop of methylated spirit put onto the hand evaporates. The hand feels cold because it provides the energy needed for the volatile liquid molecules to break free into gas.