Why does a mixed-melting-point determination work

Why does a mixed-melting-point determination work  

Effects of impurity on phase equilibria in a two-component system 

The best fail-safe way of telling whether a freshly prepared compound is identical to a sample prepared previously is to perform a mixed-melting-point experiment. 

In practice, we take two samples the hrst comprises material whose origin and purity we know is good. The second is fresh from the laboratory bench it may be pure and identical to the first sample, pure but a different compound, or impure, i.e. a mixture. We take the melting point of each separately, and call them respectively Tmelt, pure) nnd Tnieit, unknown) know for sure that the samples are different if these two melting temperatures differ. 

A mixed melting point is the only absolutely fail-safe way of determining the purity of a sample. 

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