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(1) The heating of a liquid above its boiling point (at a given pressure) or heating of a solid crystalline substance above the temperature of phase transition from one modification to another without the phase transition resulting. The transition of rhombic to monoclinic sulfur is an example of a transition from one modification to another. A superheated substance is in a labile, metastable state.
Crystalline substances cannot be superheated beyond the melting point, since all the absorbed heat goes into breaking bonds between the atoms or ions in the crystal lattice. For practically any phase transition that involves the absorption or release of heat, either slight superheating or slight supercooling must occur in order that the process occur at a finite rate. Superheated liquids are used in bubble chambers.
(2) The heating of a vapor above the saturation temperature while maintaining a constant pressure. Superheated water vapor is used widely in heat engineering.