Congruent melting occurs during

melting Melting, or fusion, is a physical process that results in the phase transition of a substance from a solid to a liquid. This occurs when the internal energy of the solid increases, typically by the application of heat or pressure, which inc ...

of a compound when the composition of the liquid that forms is the same as the composition of the solid. It can be contrasted with

incongruent melting Incongruent melting occurs when a solid substance being partially melted does not melt uniformly, so that the chemical composition of neither the resulting liquid nor the resulting solid is the same as that of the original solid. For example, mel ...

. This generally happens in two-

component Component may refer to: In engineering, science, and technology Generic systems *System components, an entity with discrete structure, such as an assembly or software module, within a system considered at a particular level of analysis * Lumped e ...

systems. To take a general case, let A and B be the two components and AB a stable solid compound formed by their chemical combination. If we draw a phase diagram for the system, we notice that there are three solid phases, namely A, B and compound AB. Accordingly, there will be three fusion or freezing point curves AC, BE and CDE for the three solid phases. In the phase diagram, we can notice that the top point D of the phase diagram is the congruent melting point of the compound AB because the solid and liquid phases now have the same composition. Evidently, at this

temperature Temperature is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making ...

, the two-component system has become a one-component system because both solid and liquid phases contains only the compound AB.Atkins' Physical Chemistry, 8th edition, by Peter Atkins and Julio De Paula Congruent melting point represents a definite temperature just like the melting points of pure components. In some phase diagrams, the congruent melting point of a compound AB may lie above the melting points of pure components A and B. But it is not always necessarily the case. There are different types of systems known in which the congruent melting point is observed below the melting points of the pure components. This happens for inter-metallic compounds, for example, for .

References

Phase transitions Materials science {{materials-sci-stub

See also

References