solute

The solute is the thing that dissolves. When you stir sugar into water, the sugar is the solute — it is the minor component that disappears into the liquid and loses its solid identity, becoming individual molecules surrounded by the liquid.

By convention the solute is the component present in the smaller amount, while the solvent is present in the larger amount; in pharmacy the solute is usually the active drug or an excipient such as a buffer salt, a preservative or a sweetener. A single solution can hold several solutes at once. For a solute to dissolve, the attractions between solute and solvent molecules must be strong enough to overcome the forces holding the solute together.

How much of a solute a solvent can hold is its solubility, and that limit depends on the chemical nature of both partners, temperature, pH and any added cosolvents or complexing agents. A useful rule of thumb is “like dissolves like”: polar solutes such as salts dissolve well in polar solvents such as water, while non-polar solutes prefer oily solvents.