What distinguishes a compound from a mixture?

• A. A mixture is a pure substance with fixed composition; a compound is not.
• B. A compound is a pure substance composed of two or more elements chemically united in fixed proportions by weight.
• C. A compound is always a mixture of elements; a mixture is a pure substance.
• D. A compound can be separated into its components by simple physical methods.

Answer: (B)

A compound is formed when elements bond chemically to create a new pure substance with a fixed composition. This means two or more elements combine in definite proportions by weight, giving the compound its own unique properties separate from the elements. Because the bonds are chemical, you can’t separate the elements by simple physical steps; you’d need a chemical change to break the bonds. In contrast, a mixture is made of substances that retain their own identities and can vary in proportions; the components can typically be separated by physical methods like filtration or distillation.

The statement that best captures this is that a compound is a pure substance composed of two or more elements chemically united in fixed proportions by weight. The other options are not correct because mixtures are not pure substances with fixed composition, a compound is not simply a mixture, and compounds aren’t separable by simple physical methods.