12.5:
Solution Equilibrium and Saturation
As a solid solute dissolves in a solvent, over time the concentration of solute molecules in the solution increases.
This increases the chances that some of the solute molecules will collide and recombine to form crystals.
Recrystallization and dissolution are two competing processes.
When the solute starts dissolving, the rate of dissolution is much greater than the rate of recrystallization. As the concentration of dissolved solute increases, the rate of recrystallization increases.
When the rate of dissolution is equal to the rate of recrystallization, a dynamic equilibrium is established.
A solution at dynamic equilibrium is known as a saturated solution. Any further solute added to this solution will remain undissolved.
Conversely, any solution containing less than the amount needed to form a saturated solution is an unsaturated solution and has the capacity to dissolve more solute.
If a saturated solution is heated, it can dissolve more solute. When the solution is cooled down slowly and left undisturbed, it forms a supersaturated solution where the solution holds more than the equilibrium amount of solute
These supersaturated solutions are unstable—meaning the excess solute often precipitates.
If a small amount of solute is added to the supersaturated solution, the crystalline solid acts as a template for the excess solute to rearrange and form crystals.
Once the newly formed crystals have settled at the bottom, the saturated solution can remain stable and dynamic equilibrium is reestablished.
12.5:
Solution Equilibrium and Saturation
Imagine adding a small amount of sugar to a glass of water, stirring until all the sugar has dissolved, and then adding a bit more. You can repeat this process until the sugar concentration of the solution reaches its natural limit, a limit determined primarily by the relative strengths of the solute-solute, solute-solvent, and solvent-solvent attractive forces. You can be certain that you have reached this limit because, no matter how long you stir the solution, undissolved sugar remains. The concentration of sugar in the solution at this point is known as its solubility.
The solubility of a solute in a particular solvent is the maximum concentration that may be achieved under given conditions when the dissolution process is at equilibrium.
When a solute’s concentration is equal to its solubility, the solution is said to be saturated with that solute. If the solute’s concentration is less than its solubility, the solution is said to be unsaturated. A solution that contains a relatively low concentration of solute is called dilute, and one with a relatively high concentration is called concentrated.
Solutions may be prepared in which a solute concentration exceeds its solubility. Such solutions are said to be supersaturated, and they are interesting examples of nonequilibrium states. For example, the carbonated beverage in an open container that has not yet “gone flat” is supersaturated with carbon dioxide gas; given time, the CO2 concentration will decrease until it reaches its solubility.
This text is adapted from Openstax, Chemistry 2e, Section 11.3: Solubility.
Suggested Reading
- Husband, Tom. "The sweet science of candymaking." ChemMatters 10, no. 11 (2014): 5-8.
- Fischinger, Andrew J. "A flotation method for growing large single crystals." Journal of Chemical Education 46, no. 8 (1969): 486.