“When a body changes from the solid to the liquid state, or from the liquid to the gaseous state, a certain amount of heat is used to accomplish this change. This heat does not raise the temperature of the body and is called latent heat. When the body changes again from the gaseous to the liquid or from the liquid to the solid state, it gives out this same quantity of heat. The latent heat of fusion is the heat supplied to a solid body at the melting point; this heat is absorbed by the body although its temperature remains essentiallyconstant during the whole process of melting. The latent heat of evaporation is the heat that must be supplied to a liquid at its boiling point to transform the liquid into a vapor. The latent heat is generally given in British thermal units per pound, or kilojoules per kilogram.”
Adapted from reference: Oberg, Erik Jones, Franklin D. Horton, Holbrook L. Ryffel, Henry H. (2012). Machinery’s Handbook (29th Edition) & Guide to Machinery’s Handbook – Specific Heat. Industrial Press.
The quantity of heat absorbed or released when a substance changes its physical phase at constant temperature (e.g. from solid to liquid at the melting point or from liquid to gas at the boiling point). The specific latent heat is the heat absorbed or released per unit mass of a substance in the course of its isothermal change of phase. The molar latent heat is the heat absorbed or released per mole of substance during an isothermal change of state.”
Reference: Daintith, John Martin, Elizabeth. (2010). Dictionary of Science (6th Edition) – latus rectum. Oxford University Press