Latent heat of vaporisation

The latent heat of vaporisation is the energy required to turn one unit mass of a liquid into gas at its boiling point, Q = m L_v. For water it is 2260 kJ/kg, nearly seven times its heat of fusion, because vaporisation pulls the molecules entirely apart rather than merely loosening them.

This large value explains everyday extremes. Evaporation cools efficiently — each gram of sweat leaving the skin carries off about 2.4 kJ — which is the basis of perspiration, evaporative coolers, and a panting dog. Conversely, condensing steam dumps the full heat back, making a 100 °C steam burn far more damaging than 100 °C water.

On a heating curve the heat of vaporisation is the tall second plateau, dwarfing the melting plateau; it is released in full when the vapour condenses.

Measured through nineteenth-century calorimetry following Black's discovery of latent heat; central to the steam-engine era and to the Clausius–Clapeyron analysis of boiling.