The change of the state of a substance from a liquid phase to a vapor is called vaporization. The reverse process i.e. the change from vapor to liquid phase is called condensation or liquefaction. The amount of heat energy require to change the phase of a unit mass of a substance from a liquid to a vapor (gas) phase at constant temperature (boiling point) is called its Specific latent heat of vaporization (Lv). It is also the energy given off when the substance changes its state from vapor to liquid phase. Mathematically:

The specific latent heat of vaporization of water = 22.6 × 10⁶J/kg = 540 Cal/g.

That is, 2.26 × 10⁶J of heat is needed to vaporize

1kg of water at 100°C to vapor at 100°C. The amount of heat needed to convert a liquid of mass m into gaseous state is, then given by: Q = mL_v.