Definition · Plain-language
Solution
A solution is a homogeneous mixture formed when one substance, the solute, dissolves evenly throughout another, the solvent.
The step most authors miss
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A uniform mixture of solute and solvent
A solution forms when a solute dissolves in a solvent to give a single, uniform phase. The solute particles — molecules or ions — spread out evenly and become so finely dispersed that they cannot be seen or filtered out, and they do not settle on standing. This uniformity is what makes a solution homogeneous: take a sample from anywhere in it and the composition is the same. In salt water, sodium and chloride ions are spread evenly among the water molecules; in air, oxygen and other gases are spread evenly through nitrogen.
Solutions are not limited to liquids
Although we picture solutions as liquids, they can form in any state. A solid solution exists when one solid is dissolved in another — alloys such as brass (zinc in copper) are solid solutions. A gaseous solution is a uniform mixture of gases, of which air is the everyday example. Liquid solutions are simply the most familiar: a solid, liquid or gas dissolved in a liquid solvent, such as sugar, ethanol or carbon dioxide dissolved in water. The defining feature in every case is uniform mixing without chemical bonding.
Concentration and saturation
How much solute a solution contains is its concentration. A dilute solution holds little solute per unit of solvent; a concentrated one holds much more. There is usually a limit: when no more solute will dissolve at a given temperature, the solution is saturated. Heating often lets a solvent dissolve more solute, which is why a saturated solution can sometimes hold extra solute when warm and then deposit crystals as it cools. Because solutions are physical mixtures, the solute can be recovered — for instance by evaporating the solvent away.
Key facts
At a glance
- Definition: a homogeneous mixture of a solute dissolved in a solvent
- Appearance: uniform; particles do not settle or filter out
- Components: solute (dissolved) and solvent (does the dissolving)
- States: liquid, solid (alloys) or gaseous (air)
- Separation: by physical means, e.g. evaporation
- Concentration: dilute, concentrated or saturated
Common misconceptions
What people often get wrong
Often heard: A solution must be a liquid.
Actually: Solutions can be solid, liquid or gaseous. Alloys such as brass are solid solutions and air is a gaseous solution. The defining feature is uniform mixing without chemical bonding, not the state of matter.
Often heard: In a solution the solute is chemically bonded to the solvent.
Actually: A solution is a physical mixture, not a compound. The solute disperses among the solvent particles without forming new chemical bonds, which is why it can be separated again by physical methods.
Often heard: A solution can dissolve an unlimited amount of solute.
Actually: There is usually a limit. Once a solution is saturated, no more solute will dissolve at that temperature. Heating often raises the limit, but it is not infinite.
Going deeper
