A substance has a melting point of 100°C and a boiling point of 150°C. If a sample of this substance melts at 98°C and boils at 148°C, is the substance pure or impure? Justify your answer.

A high-scoring answer should explain interpret melting point or boiling point data to decide whether a substance is pure or impure. 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.