Question

Is a substance that melts or boils at a fixed temperature a pure substance?

Answer 1

yu

Answer 2

yes they do. Impurities always reduce melting points as they get in the way of molecular arrangement in a solid.
If there is a known substance when heated to a certain point the bonds will break and it will melt. The same amount of energy is required to break molecular bonds, and so it will have a fixed melting point.