What is meant by a pure substance in chemistry, and how does it relate to melting and boiling points?

A high-scoring answer should explain explain that a pure element or compound melts and boils at specific temperatures. 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.