What is the Difference Between Congruent and Incongruent Melting Point?

The main difference between congruent and incongruent melting points lies in the composition of the liquid that forms during the melting process.

• Congruent Melting Point: This occurs when a phase melts to a liquid with the same composition as the solid. Congruent melting is observed in systems with two components, A and B, where a stable solid compound (AB) is formed by their chemical reaction. The congruent melting point represents a definite temperature, just like the melting points of pure substances. An example of a system with a congruent melting point is the magnesium-zinc system (Mg(Zn)2).
• Incongruent Melting Point: This occurs when a phase melts to a liquid with a composition different from the solid and produces a solid of different composition to the original solid. Incongruent melting is observed when a solid substance does not melt uniformly, decomposing into another solid and a liquid with different compositions. An example of a system with an incongruent melting point is the sodium chloride-water system.

In summary, congruent melting points involve melting to a liquid with the same composition as the solid, while incongruent melting points involve melting to a liquid with a different composition and producing a solid of different composition to the original solid.

Comparative Table: Congruent vs Incongruent Melting Point

The difference between congruent and incongruent melting points lies in the behavior of substances during the transition from solid to liquid. Here is a summary of their characteristics:

Congruent and incongruent melting points usually happen in two-component systems. An example of incongruent melting is the sodium chloride-water system, where sodium chloride (NaCl) in association with two water molecules forms NaCl·2H2O.