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 thi…
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.
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Q1: What is the difference between a saturated and unsaturated solution?
A saturated solution contains the maximum concentration of dissolved solute at equilibrium, where dissolution and recrystallization rates are equal. An unsaturated solution has less solute than needed to reach saturation and can dissolve more solute. The concentration limit of a saturated solution is determined by the relative strengths of solute-solute, solute-solvent, and solvent-solvent attractive forces.
Q2: What happens when you heat a saturated solution?
Heating a saturated solution increases its capacity to dissolve more solute. When the heated solution is cooled slowly and left undisturbed, it can form a supersaturated solution, which holds more solute than the equilibrium amount. Supersaturated solutions are unstable and the excess solute often precipitates out.
Q3: How does dynamic equilibrium establish in a solution?
As a solid solute dissolves, the concentration of dissolved molecules increases, raising the chance of recrystallization. Initially, dissolution occurs faster than recrystallization. When the rate of dissolution equals the rate of recrystallization, dynamic equilibrium is established, creating a saturated solution where both processes continue at equal rates.
Q4: What is a supersaturated solution and why is it unstable?
A supersaturated solution contains more dissolved solute than the equilibrium amount, making it a nonequilibrium state. These solutions are unstable because the excess solute tends to precipitate. Adding a small amount of solute or disturbing the solution provides a template for crystallization, allowing the excess solute to rearrange and form crystals.
Q5: What determines the solubility of a solute in a solvent?
Solubility is determined primarily by the relative strengths of solute-solute, solute-solvent, and solvent-solvent attractive forces. These intermolecular forces govern the maximum concentration achievable when the dissolution process reaches equilibrium. The solubility represents the limit at which no more solute can dissolve under given conditions.
Q6: Why does recrystallization increase as solute concentration rises?
As dissolved solute concentration increases, the probability of solute molecules colliding and recombining to form crystals increases. Higher concentration means more solute particles in solution, raising collision frequency. This increased recrystallization rate eventually matches the dissolution rate, establishing the dynamic equilibrium characteristic of a saturated solution.
Q7: What happens to a supersaturated solution when a seed crystal is added?
Adding a seed crystal to a supersaturated solution provides a template for crystallization. The excess solute rearranges around the crystalline solid and forms new crystals. Once the crystals settle, the solution returns to saturation and dynamic equilibrium is reestablished, stabilizing the solution.