Phase changes

Phase Change Energy Calculator

Calculate the latent energy associated with melting, freezing, vaporisation or condensation using Q = mL. The calculation applies only to the phase-change portion of a process, not to heating or cooling before or after it.

Calculate latent energy

Why temperature can pause while energy still moves

During an equilibrium phase transition of a pure substance at fixed pressure, energy can enter or leave while the temperature remains approximately constant. The energy changes the phase fraction rather than simply increasing or decreasing molecular kinetic energy.

Melting and vaporisation absorb energy

Melting requires energy to convert a solid into a liquid. Vaporisation requires energy to convert a liquid into vapour. For the same mass, vaporisation commonly involves a much larger latent-energy change than melting.

Freezing and condensation release energy

The reverse phase changes release the corresponding latent energy to the surroundings. This is why condensing steam can transfer substantial energy even while remaining near its saturation temperature.

Latent heat is only one part of a full heating problem

If ice starts below its melting point and ends as steam above the boiling point, the total energy must include several stages: sensible heating of ice, fusion, sensible heating of liquid water, vaporisation, and possibly sensible heating of vapour. This calculator evaluates only one selected phase-change stage.

Reference value context

The default 2256.4 kJ/kg is a representative value for the enthalpy of vaporisation of water near its normal boiling point. Property values vary with temperature and pressure, so use data appropriate to the actual condition when precision matters.