Question

what is the best defnition 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. It is made up of only one type of component (either an element or a compound). The option "changes that make new molecules" refers to a chemical reaction, not a pure substance. The option "a substance that is mixed with another" describes a mixture, not a pure substance.

Answer:

a substance made of only one component