A compound is defined as

• A. Two or more different elements bound together
• B. A single element
• C. A homogeneous mixture
• D. An alloy with constant composition

Answer: (C)

Substances that are compounds are formed when two or more elements are chemically bound in a fixed ratio. That chemical bonding creates a new substance with properties different from the elements themselves, and the composition — the proportions of elements — is fixed. A single element isn’t a compound because there’s nothing to bond to form a new substance.

A homogeneous mixture, by contrast, is just physically blended components where the individual substances retain their own identities, and their proportions can vary from sample to sample. You can often separate them by physical methods, not by breaking chemical bonds.

An alloy is a uniform solid mixture of elements (usually metals) with a definite overall composition, but it isn’t formed by the creation of new chemical bonds that yield a discrete molecular formula the way a compound does. It’s best viewed as a homogeneous material arising from mixing elements, rather than a substance defined by a chemical formula.

So, the defining idea is chemical bonding between elements in a fixed ratio to form a new substance.