A chemical combination of matter in definite (fixed) proportions is a _____.

• A. mixture
• B. solution
• C. element
• D. pure substance

Answer: (D)

Fixed-composition substances are pure substances. A pure substance has a definite, unchanging composition, which means every sample of it has the same ratio of its components. A chemical combination in a definite proportion describes a compound, which is a type of pure substance made from two or more elements bonded together in a specific ratio. That’s why this choice is best phrased as a pure substance: it signals that the material is not a mixture of different substances, and its composition doesn’t vary from sample to sample.

Mixtures lack fixed proportions and can be separated by physical means. A solution is a homogeneous mixture, not a pure substance. An element is itself a pure substance, but it consists of only one type of atom and doesn’t represent a chemical combination of multiple elements.