Water Heat of Vaporization vs Temperature Charts Tables Calculator

The (latent) heat of vaporization (ΔH_vap), also known as the enthalpy of vaporization or enthalpy of evaporation, is the amount of energy that must be added to a liquid substance to transform a given quantity of it into a gas at constant pressure. For water at atmospheric pressure (100 °C / 212 °F), this is approximately 2257 kJ/kg (970 Btu/lbm). This energy goes entirely into overcoming the intermolecular hydrogen bonds of water — none of it raises the temperature.

The enthalpy of vaporization is a function of the pressure at which the phase change occurs. At higher pressures, water boils at a higher temperature, and the distinction between liquid and vapor diminishes — the molecules are closer together and hold less additional energy at the phase boundary. As a result, ΔH_vap decreases monotonically with rising temperature, from about 2501 kJ/kg at 0 °C to 0 at the critical point. This has direct implications for steam power cycles: the thermal efficiency of a steam turbine depends on the enthalpy drop across the turbine, which is linked to the heat of vaporization at operating pressure.

The heat of vaporization vanishes completely at the critical point, where the liquid and vapor phases become indistinguishable. For water:

Critical temperature: 373.946 °C  /  705.103 °F
Critical pressure: 220.6 bar  =  22.06 MPa  =  3200 psi

Above the critical temperature, water exists only as a supercritical fluid — no amount of pressure will cause it to condense into a distinct liquid phase, and no heat of vaporization applies.