Which statement best distinguishes a mixture from a solution?

• A. A mixture can be separated by physical means; a solution is a homogeneous mixture at the molecular level.
• B. A mixture is always homogeneous; a solution is heterogeneous.
• C. A mixture contains only one substance; a solution contains multiple substances.
• D. A solution cannot be separated by physical methods.

Answer: (C)

The key idea is how uniformly the substances are distributed. A solution is a homogeneous mixture where the solute is dissolved in the solvent, so every sample has the same composition at the molecular level. You can’t pick out the different parts by looking, and the mixture behaves as a single substance in terms of properties.

Mixtures, on the other hand, can be either homogeneous or heterogeneous. In a heterogeneous mixture you can see different components or have them separated physically (like sand in water or oil and water). Even some mixtures that look uniform aren’t true solutions because the components aren’t dissolved and can be separated more easily by physical means.

So, the distinction hinges on uniform, molecular-level mixing in a solution versus the possible nonuniform mixing in a general mixture. The idea that a mixture contains only one substance isn’t accurate, since mixtures are made of two or more substances.