How is a solution defined in chemistry?

A solution is defined in chemistry as a homogeneous mixture of two or more substances. This means that in a solution, the components are uniformly distributed at the molecular level, resulting in a consistent composition throughout the mixture. For instance, when salt is dissolved in water, sodium and chloride ions become evenly dispersed within the water, creating a clear solution that does not have distinct layers.

In the context of the other options: a pure substance made of a single element does not represent a mixture, a characteristic necessary for a solution. A mixture that can settle into layers over time indicates heterogeneity, as seen in suspensions or emulsions, where different phases can separate over time rather than remaining uniformly mixed. Lastly, a compound that does not dissolve in water would not fit the definition of a solution, as solutions require at least one solute to be soluble in a solvent, thereby establishing a homogenous mixture. Thus, the correct option precisely describes the nature of solutions in chemistry.