Incongruent transition, in chemistry

Chemistry

Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....

, is a mass transition between two phases

Phase (matter)

In the physical sciences, a phase is a region of space , throughout which all physical properties of a material are essentially uniform. Examples of physical properties include density, index of refraction, and chemical composition...

 which involves a change in composition. This is contrasted with congruent transition, for which the composition remains the same.

The transition may be that of melting

Melting

Melting, or fusion, is a physical process that results in the phase change of a substance from a solid to a liquid. The internal energy of a substance is increased, typically by the application of heat or pressure, resulting in a rise of its temperature to the melting point, at which the rigid...

, vaporization

Vaporization

Vaporization of an element or compound is a phase transition from the liquid or solid phase to gas phase. There are three types of vaporization: evaporation, boiling and sublimation....

 or allotropism

Allotropy

Allotropy or allotropism is the property of some chemical elements to exist in two or more different forms, known as allotropes of these elements...

. The concept is also often extended to related phenomena, for example, incongruent dissolution of a solid by a liquid solvent, which is often encountered in geology

Geology

Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...

.

The term "phase decomposition" is sometimes used to describe incongruent transition. However, it has to be kept in mind that incongruent transition is described by an equilibrium

Chemical equilibrium

In a chemical reaction, chemical equilibrium is the state in which the concentrations of the reactants and products have not yet changed with time. It occurs only in reversible reactions, and not in irreversible reactions. Usually, this state results when the forward reaction proceeds at the same...

.

For an example, see incongruent melting

Incongruent melting

Incongruent melting occurs when a substance does not melt uniformly and decomposes into another substance. For example, potassium feldspar decomposes to leucite when it melts. The decomposition is not complete, however...

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