Which of the following is not a pure substance?

• A. Compound
• B. Element
• C. Mixture
• D. Isotope

Answer: (C)

Pure substances have a single chemical identity and a fixed composition. Elements are pure substances because they are made from one type of atom. Isotopes are still the same element, just atoms with a different number of neutrons, so chemically they remain the same substance. Compounds are also pure substances formed when elements join in fixed ratios with chemical bonds, giving a new substance with its own definite composition.

A mixture, however, contains two or more substances physically combined. Its composition can vary and you can separate the components by physical methods, so it doesn’t have the single, fixed identity of a pure substance.

So, the material that is not a pure substance is a mixture.