What characterizes a pure substance in terms of its melting point?

A pure substance is characterized by having a defined melting point, which is a specific temperature at which it transitions from a solid to a liquid. This melting point is consistent and reproducible for a given substance under controlled conditions, meaning that every sample of that pure substance will melt at the same temperature. This property is a distinctive feature of pure substances and allows chemists to identify and characterize materials based on their melting points.

In contrast, a mixture or an impure substance does not have a single, definite melting point; instead, it typically exhibits a range of temperatures over which the melting occurs, reflecting the presence of multiple components with different melting characteristics. The idea of surrounding materials influencing melting points also applies more to mixtures than to pure substances.