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a solution in stable equilibrium with an excess of the solute under given conditions (temperature and pressure). Examples are an aqueous solution of a salt in which crystals of the salt are present and a solution of a gas in water through which the same gas is being passed.
The concentration of a saturated solution—that is, the content of the solute in it—is called the solubility of the solute at a given temperature and pressure. In a given solvent under fixed conditions, this concentration may be very large for readily soluble substances but very low for poorly soluble substances. A solution containing a quantity of solute that is less than the concentration of the saturated solution under the given conditions is called unsaturated. In the absence of crystals of the solute, cooling of saturated solutions may not lead to crystallization, in which case a supersaturated solution results. A supersaturated solution contains more solute than a saturated solution. The introduction of a crystal of the solute into such a solution leads to precipitation of crystals from the solution. (For a discussion of saturated solutions in systems with limited mutual solubility, seeLIQUID MIXTURES and SOLID SOLUTIONS.)