Question

how is the melting point affected by impurities present in a solid substance

Answer 1

EFFECT OF IMPURITIES ON FREEZING POINT:
When impurity is added , Its freezing point decreases.
Reason:
The depression in Freezing point increases with increase in concentrate of solute because on adding the solute, vapour pressure becomes  lower than that of pure solvent.
Hence freezing occurs at lower temperature.
Example:
Freezing point of water is 0⁰C under normal atmospheric pressure. If we add salt, the freezing point decreases .