What is a solution?

A solution is best defined as a homogeneous mixture in which substances dissolve completely, resulting in a mixture that has a uniform composition throughout. This means that when one substance, such as salt, mixes with another, such as water, the salt breaks down into its individual ions and becomes evenly distributed within the water. The resulting mixture does not have distinct layers or parts that are visibly different, which is characteristic of a homogeneous mixture.

In contrast, other options describe different forms of mixtures or substances. A mixture where substances settle out refers to a suspension, which is not uniform in composition and can have layers. A heterogeneous mixture contains visibly different substances or phases that can be distinguished easily and are not uniformly distributed. Lastly, a purely single element refers to a substance made up of only one type of atom, which does not qualify as a mixture at all. This distinction is crucial when identifying types of matter in physical science.