Compound definition.

• A. A pure substance consisting of a single element.
• B. A pure substance composed of two or more elements chemically united in fixed proportion by weight.
• C. A mixture that can be separated into its components by physical methods.
• D. A chemical reaction that forms a new compound.

Answer: (B)

A compound is a pure substance made from two or more elements that are chemically bonded in fixed proportions by weight. The chemical bonds create a new substance with properties distinct from the individual elements, and the composition is always the same in any sample of that compound. For example, water is always two hydrogens for each oxygen, and table salt is always sodium chloride in a 1:1 ratio, regardless of where you obtain it.

This differs from a pure element, which is just a single kind of atom, and from a mixture, where the components are combined physically and can be separated by physical methods and do not have fixed ratios. A chemical reaction that forms a compound describes the process of making a compound, not the definition of the compound itself.