Which statement best distinguishes a mixture from a pure substance?

Which statement best distinguishes a mixture from a pure substance?

• A. A mixture contains two or more substances that can be physically separated; a pure substance has a single type of particle.
• B. A mixture has a fixed composition; a pure substance can vary in composition.
• C. A mixture cannot be separated into simpler substances; a pure substance can.
• D. A mixture has a single type of particle; a pure substance contains two.

Answer: (A)

Mixtures are made of two or more substances that can be physically separated, while pure substances consist of only one kind of particle. That means a mixture can be separated into its components by physical methods like filtering, distilling, or magnetizing, because the substances are not chemically bonded to each other. A pure substance has a single type of particle—either a single element or a compound with a fixed ratio of elements—and its composition is consistent throughout, so it cannot be broken down into simpler substances by simple physical processes. This is why the statement describing mixtures as containing two or more substances that can be physically separated, and pure substances as having a single type of particle, best captures the distinction. The other ideas—such as mixtures having fixed, unchanging composition, or mixtures that cannot be separated, or mixtures with a single particle type—don't fit the actual features of mixtures and pure substances.

Mixtures are made of two or more substances that can be physically separated, while pure substances consist of only one kind of particle. That means a mixture can be separated into its components by physical methods like filtering, distilling, or magnetizing, because the substances are not chemically bonded to each other. A pure substance has a single type of particle—either a single element or a compound with a fixed ratio of elements—and its composition is consistent throughout, so it cannot be broken down into simpler substances by simple physical processes.

This is why the statement describing mixtures as containing two or more substances that can be physically separated, and pure substances as having a single type of particle, best captures the distinction. The other ideas—such as mixtures having fixed, unchanging composition, or mixtures that cannot be separated, or mixtures with a single particle type—don't fit the actual features of mixtures and pure substances.