Which term describes a single type of matter with fixed properties and composition?

• A. Heterogeneous mixture
• B. Elements
• C. Compounds
• D. Pure substance

Answer: (D)

Think about matter that has a uniform, unchanging composition. A pure substance has fixed properties because its composition doesn’t vary from one sample to another. It can be made of a single type of particle, like the atoms in an element, or of molecules formed by two or more elements in a fixed ratio, like a compound. The phrase “single type of matter” without any variety in the mixture points to this idea: there’s no blend of different substances. That’s why the term that best fits is pure substance.

Elements are a kind of pure substance, as are compounds, but the overall label that describes any single, uniform kind of matter with fixed composition is pure substance. In contrast, a heterogeneous mixture would have different parts with different properties, so it isn’t a pure substance.