溶液の平衡と飽和

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.