Solution

A solution is a material composed of more than one component, where the value of the phase_of_matter property depends on composition. Its components can still be separated by methods based on differences in physical features.

A solution sits awkwardly between substance and mixture, and that is the point of giving it its own name. From across the room a solution can look like a substance: pour a glass of salt water and the eye finds nothing but liquid; nothing settles, nothing separates, the property values look uniform throughout. Sample it carefully and the description shifts. There is more than one component in the glass. The salt is in the water; the water is around the salt. They do not announce themselves the way sand and pebbles do, but they are there, and they can be brought back out.

A solution has all the features of the material class. It is composed of more than one component, like a mixture, and like a mixture its components are separable by methods that exploit differences in physical features. What sets it apart is the phase_of_matter property. For a substance the value is one; for a mixture the value is greater than one; for a solution the value depends on composition. Change how much of one component is mixed with how much of another and the phase value can change.

This is why solution earns a class of its own rather than living quietly inside the mixture class. The classification of materials is by composition, and a class whose phase value moves with composition is doing something the substance and mixture classes do not. A learner picking up a piece of the world to ask "what is this," should learn the solution test alongside the substance and mixture test, because solutions are common and they are easy to misread as something simpler than they are.