Saturated and Supersaturated Solutions
We often find that there is a limit to the quantity of soluteThe substance added to a solvent to make a solution. which will dissolve in a given quantity of solventThe substance to which a solute is added to make a solution.. This is especially true when solids dissolve in liquids. For example, if 36 g KCl crystals is shaken with 100 g H2O at 25°C only 35.5 g of the solid dissolves. If we raise the temperatureA physical property that indicates whether one object can transfer thermal energy to another object. somewhat, all the KCl will dissolve, but on cooling to 25°C again, the extra 0.5 g KC1 will precipitate, leaving exactly 35.5 g of the saltAn ionic compound that can be formed by replacing the hydrogen ion of an acid with a different cation. dissolved. We describe this phenomenon by saying that at 25°C the solubilityThe extent to which a solute dissolves in a solvent; often expressed as the mass of a substance that will dissolve in 100 mL of solvent. of KCl in H2O is 35.5 g KC1 per 100 g H2O. A solutionA mixture of one or more substances dissolved in a solvent to give a homogeneous mixture. of this composition is also described as a saturatedDescribes 1) a solution that contains the equilibrium concentration of a solute, or 2) an organic compound that contains no double or triple bonds (such as an alkane). solution since it can accommodate no more KCl.
Under some circumstances it is possible to prepare a solution which behaves anomalously and contains more solute than a saturated solution. Such a solution is said to be supersaturatedDescribes a solution that has a greater concentration of a solute than under equilibrium conditions.. A good example of supersaturation is provided by Na2S2O3, sodium thiosulfate, whose solubility at 25°C is 50 g Na2S2O3 per 100 g H2O. If 70 g Na2S2O3 crystals is dissolved in 100 g hot H2O and the solution cooled to room temperature, the extra 20 g Na2S2O3 usually does not precipitate. The resulting solution is supersaturated; consequently it is also unstable. It can be “seeded” by adding a crystal of Na2S2O3, whereupon the excess salt suddenly crystallizes and heatEnergy transferred as a result of a temperature difference; a form of energy stored in the movement of atomic-sized particles. is given off. After the crystals have settled and the temperature has returned to 25°C, the solution above the crystals is a saturated solution—it contains 50 g Na2S2O3.
Another example of crystallizing salt out of a supersaturated solution can be seen in the following video. In this case, a supersaturated solution of sodium acetate is poured over a crystals of sodium acetate. These crystals provide the latticeThe points in space that define the ordered, repeating arrangement of atoms, ions, or molecules in a crystal. structure "seed" which causes the sodium acetate ions in solution to crystallize out.
The video begins with a few crystals of sodium acetate placed on the lab bench. A supersaturated solution of sodium acetate is poured over the crystals providing a seed or crystallization. The salt begins to crystallize, forming a large sodium acetate structure from the precipitationThe formation of a solid within a solution, often by the combination of cations and anions to form an insoluble ionic compound. of the ions out of solution. When the sodium acetate crystallizes, the oppositely charged ions are brought closer together by the crystal structure. Since formation of a crystal lattice lowers potential energyA system's capacity to do work. by placing like charges close together, the system releases the excess energy in the crystallization process. Thus, the structure ends up being warm to the touch from this excess energy.