A compound is defined as

• A. Two or more elements chemically combined
• B. A mixture of substances that are not chemically combined
• C. A single element
• D. A pure substance made of molecules

Answer: (A)

A compound is formed when two or more elements join together by chemical bonds in a definite, fixed ratio, creating a new substance with properties different from the elements that make it. This chemical bonding and fixed composition distinguish compounds from mixtures, which are just physical blends of substances that can be separated without chemical reactions.

So, the statement that best describes a compound is that it consists of two or more elements chemically combined. The other descriptions describe mixtures (not chemically bound), a single element (not a compound), or a vague notion of a “pure substance made of molecules” (which can apply to some elements as well as compounds, and thus isn’t specific to compounds).