Phase changes involve energy changes. Ice, for example, requires heat for melting. Normally this melting takes place at constant pressure (atmospheric pressure) and during phase change, temperature remains constant (at 273 K).

H₂O (s) → H₂O (l);

Δ_fusH^Θ = 6.00 kJ mol^-1

Here Δ_fus H^Θ is enthalpy of fusion in standard state. If water freezes, then process is reversed and an equal amount of heat is given off to the surroundings, that is

Δ_freezeH^Θ = -Δ_fus H^Θ

This enthalpy is called enthalpy of fusion or molar enthalpy of fusion.

Δ_fus H^Θ = H^Θ_liquid - H^Θ_solid

Melting of a solid is endothermic, so all enthalpies of fusion are positive.

Amount of heat required to vaporize one mole of a liquid at constant pressure and temperature is called its enthalpy of vaporization or molar enthalpy of vaporization. Here constant temperature is its boiling point, T_P.

In case of water vaporization

H₂O (l) → H₂O (g); Δ_vapH^Θ = + 40.79 kJ mol^-1

Sublimation is a direct conversion of a solid into its vapour. Solid CO₂ or 'dry ice' sublimes at 195 K with Δ_subH^Θ = 25.2 kJ mol^-1, napthalene sublimes slowly and for this Δ_subH^Θ = 73.0 kJ mol^-1.

Because enthalpy is a state property, the enthalpy of sublimation can be expressed as

Δ_subH^Θ = Δ_fusH^Θ + Δ_fusH^Θ.