Question

The substance that is composed of two or more elements that are chemically combined with one another in a fixed proportion by mass is called ___

Answer 1

Any substance that contains only one kind of an atom is known as an element. Elements combine to form compounds that are often divided into two categories. Metals often react with nonmetals to form ionic compounds.

Example: a compound called calcium carbonate with the formula ${\text{CaCO}}_3$ has a fixed proportion by mass of calcium, carbon and oxygen (in 10:3:12 ratio). Any proportion that is different from this is no longer called calcium carbonate because the chemical nature of the compound changes.