Question

what is the best definition of a pure substance? a substance made of only one component changes that make new molecules a substance that is mixed with another

Explanation:

A pure substance is defined as a material that has a constant composition and consistent properties throughout. A substance made of only one component fits this definition as it has uniformity. Changes that make new molecules refer to chemical reactions, not the definition of a pure substance. A substance mixed with another is a mixture, not a pure substance.

Answer:

a substance made of only one component