Heat guide
Latent Heat and Phase Change
Latent heat is the energy absorbed or released during a phase change without a corresponding temperature change under the idealized constant-pressure condition.
Sensible heat and latent heat
Sensible heating changes temperature and is commonly modelled with Q = mcΔT. Latent heating changes phase and is commonly modelled with Q = mL, where L is the appropriate latent heat.
Why temperature can pause
During melting or boiling, added energy goes into changing molecular arrangement rather than increasing average molecular kinetic energy in the same way as sensible heating. The temperature can therefore remain nearly constant while the phase transition proceeds.
Fusion and vaporization
Latent heat of fusion describes solid-liquid phase change. Latent heat of vaporization describes liquid-vapour phase change. Vaporization generally requires much more energy because molecules must separate much more extensively.
Real materials are more complicated
Pure substances under controlled pressure have well-defined phase-change temperatures. Mixtures can melt or boil over ranges. Pressure also changes boiling temperature, and many materials decompose before a clean phase transition occurs.
Energy accounting across several stages
A complete heating problem may require multiple terms: sensible heating to the phase-change temperature, latent energy for the phase change, then additional sensible heating in the new phase.
Latent heat does not mean temperature is irrelevant
During an ideal equilibrium phase change at fixed pressure, added or removed energy can change phase without changing temperature. Real systems can depart from that simple picture because pressure varies, mixtures transform over temperature ranges, and temperature gradients exist within the material and surrounding equipment.
Latent-heat calculations are therefore energy balances under stated conditions. They should not be read as a claim that every part of a boiling, freezing or melting system remains at one exact temperature.
Phase diagrams show why latent heat is condition-dependent
The temperature and pressure at which phases coexist are related by the substance’s phase diagram. Melting and boiling temperatures therefore shift when pressure changes, and the latent heat associated with the transition also changes with state. Near a critical point, the distinction between liquid and vapour eventually disappears and the latent heat of vaporisation approaches zero.
For ordinary calculations over a narrow range, a tabulated latent heat at the relevant pressure is usually sufficient. Wide-range or precision work should use thermodynamic property data matched to the actual state.
Worked example: melting ice
Using a representative latent heat of fusion for water near 0 °C of about 333.5 kJ/kg, melting 0.50 kg of ice already at its melting temperature requires about 167 kJ in the ideal constant-pressure calculation. That energy changes phase rather than raising the temperature of the ice-water mixture during the transition.
If the ice begins below 0 °C, energy must first warm the ice. If the resulting water must then be heated above 0 °C, a separate sensible-heating term follows the melting term.
Pressure changes phase-change conditions
Boiling occurs when liquid vapour pressure is compatible with the surrounding pressure, so reducing pressure lowers the boiling temperature of water. This is why high-altitude cooking and vacuum processing differ from sea-level conditions.
Latent heat itself can also vary with temperature and pressure. Reference values are therefore conditions-dependent rather than exact constants for every process.
Condensation can deliver large heat transfer
When vapour condenses, it releases latent energy. Steam contacting a cooler surface can therefore transfer substantial energy even when the temperature difference is modest. This principle appears in steam heating, condensers and food steaming.
The actual rate depends on how condensate forms and drains, surface condition, pressure and other transport effects, so latent energy per kilogram should not be confused with heat-transfer rate.
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Sources and further reading
Use the linked primary or authoritative resources for additional detail, standards and source-specific conditions.