Question

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

Answer 1

Elements combine to form compounds. For 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.