What is a pure substance?

• A. One substance
• B. A mixture of two or more substances
• C. A substance that can be separated into simpler substances by physical means
• D. A substance composed of two or more elements chemically bonded

Answer: (A)

A pure substance is a single kind of material with uniform composition and properties throughout. That means the sample is not a mixture of different substances, and it cannot be separated into other substances by physical means; you’d need a chemical change to break it down. Pure substances include elements, like oxygen, and compounds, like water, which form when elements bond chemically. In this sense, describing a pure substance as “one substance” fits the idea that there’s only a single material present, unlike mixtures. The other ideas point to mixtures (two or more substances together) or to a way of splitting substances that applies to mixtures, while compounds are indeed pure substances, but the broad and most straightforward description is that a pure substance is one substance.