When discussing solutions, the solute is the substance which dissolves, while the solvent is the substance into which the solute dissolves. Creating a salt water solution involves dissolving salt in water, making salt the solute and water the solvent. However, determining which is the solute or solvent is not always so simple, and some rules exist for determining which substance dissolves in the other.
Three common states of matter exist: gases, liquids and solids. Only the solute goes through a change in state when a solution is created. If both reagents remain in the same state, the one of which there is the least remaining is the solute.
One example of a solution that is somewhat more complicated is the dissolution of hydrogen chloride gas within a liquid to create hydrochloric acid. Dissolving a gas in a solid, as with hydrogen in platinum, is a possible solution. While one might view a liquid as the solvent when it enters a solution with a solid, that is not always the case. It is possible to dissolve liquid mercury in solid sodium to create an alloy. Wine that is 12 percent alcohol by volume indicates the ratio of the solution of the alcohol within the solvent water.