How do impurities affect the melting point of a substance?

A high-scoring answer should explain explain that impurities change the melting point and boiling point of a substance. Use method first, observation second, conclusion last: name the relevant test or measurement, state the observation, and then connect the result to Pure substances.

This is correct because single element or compound / fixed melting point is the evidence expected for pure substance, and purity evidence is the result that supports the conclusion. A complete answer should use the approved objective wording, include the relevant evidence, and avoid unsupported identification claims.