A change from one physical state to another (i.e. solid, liquid, and gas). The transition is accompanied by a change in energy. While heat is absorbed or evolved, there is no temperature change. Examples include the melting of a solid to a liquid, the freezing of a liquid to a solid, the boiling or evaporation of a liquid to a vapour, the condensation of a vapour to a liquid, and the sublimation of a solid to a vapour.