Congruent melting occurs during 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...

 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

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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The "congruent melting point" is the melting point

Melting point

The melting point of a solid is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depends on pressure and is usually specified at standard atmospheric pressure...

(the pressure and temperature) when the composition of the liquid formed is the same as the composition of the solid.