why does the presence of impurities lower the melting point of a solid?

The presence of an impurity in the molten compound

(i) Reduces vapor pressure thus lowering the melting point of the compound.

 (ii) Broaden or depression in the melting point range. The melting point is not anymore sharp, but a range of values. The more impurities present, the greater the range of values.

This is because impurities disturb the consistency and organization of the crystal lattice at the molecular level. Impurities don’t “fit” correctly into what would be the normal pure lattice.

The disturbance weakens the lattice, so that the lattice can be broken down more easily; the weakened structure melts more easily at reduced temperature (depression).

Disturbance of the lattice makes it non-uniform. At the molecular level, the molecules closest to the impurities melt fastest.

Further away from the impurities, the crystal lattice is relatively undisturbed and therefore melts at or nearer the normal temperature. Thus the melting point is deviated.