Latent heat
The heat absorbed or released during a phase change at constant temperature — energy spent on bonds, not on warming.
Definition
Latent heat is the energy a substance absorbs or releases as it changes phase, with no accompanying change in temperature. The word means 'hidden': the heat does not raise the thermometer because it goes into rearranging or breaking molecular bonds rather than into molecular motion. It breaks the simple Q = mcΔT picture, since here ΔT = 0 while Q is large.
Each transition has its own latent heat per unit mass — the latent heat of fusion for melting, of vaporisation for boiling, of sublimation for solid-to-gas — so that Q = mL. The energy is released again on the reverse transition: freezing gives back the heat of fusion, condensation the heat of vaporisation.
Latent heat explains why ice is an effective coolant, why sweat and evaporation cool, why steam burns are severe, and why melting and boiling appear as flat plateaus on a heating curve. It also enters the Clausius–Clapeyron relation, which links the latent heat to the slope of the coexistence curve.
History
Discovered by Joseph Black in the 1760s, who observed that ice absorbs heat while melting without changing temperature, and who used the idea to advise James Watt on the steam engine.