- When solid solute (substance or particles) and liquid solvent are mixed, the only reactions are dissolution or crystallization.
- A dissolution is the dissolving process of the solid solute.
- A crystallization is the opposite, which causes the solid solute to remain undissolved.
Types of Saturation
|Kinds of Saturation ||Definition |
|Saturated Solution ||A solution with solute that dissolves until it is unable to dissolve anymore, leaving the undissolved substances at the bottom. |
|Unsaturated Solution ||A solution (with less solute than the saturated solution) that completely dissolves, leaving no remaining substances. |
|Supersaturated Solution ||A solution (with more solute than the saturated solution) that contains more undissolved solute than the saturated solution because of its tendency to crystallize and precipitate. |
Example 1: This is an example of a saturated solution. In Figure 1.1-1.3, there is a constant amount of water in all the beakers. Figure 1.1 is the starting process of saturation where the solid solute starts to dissolve (represented by red arrows). In the next beaker, Figure 1.2, it is noticeable that a lot of the solid solute has dissolved (represented by red arrows), but not completely for the reason that the process of crystallization (represented by blue arrows) has begun. By the last beaker, Figure 1.3, only a small amount of the solute solvent remains undissolved. In this process, the rate of the crystallization is stronger than the rate of dissolution, causing the amount of dissolved to be less than the amount crystallized.
Example 2: This is an example of an unsaturated solution. In Figure 2.1-2.3, there is a constant amount of water in all the beakers. Figure 2.1 is the starting process of solid solute beginning to dissolve (represented by red arrows). In the next beaker, Figure 2.2, there is an obvious difference that a large amount of the solute has dissolved. The size of the red arrows are much larger than those of the blue arrows, which means that the rate of dissolution is much greater than rate of crystallization. By the last beaker, Figure 2.3, the solute solvent has completely dissolved in the liquid solvent.
Example 3: This is an example of a supersaturated solution. In Figure 3.1-3.3, there is a constant amount of water in all the beakers. Figure 3.1 shows a beaker with more solid solute than the saturated solution (Figure 1.1) dissolving. By Figure 3.2, crystallization begins and causes the solid to crystallize as it slowly decreases the rate of dissolution. By the last picture, Figure 3.3, the solids become a crystallized formed which begins to harden.
Factors Affecting Saturation
- solubilities of ionic solutions increase with an increase in temperature with exceptions of compounds containing anions.
- finely divided solids has a higher solubility rate.
- while the solubility rate depends primarily on temperature, the rate of crystallization depends on the concentration of the solute at the crystal surface.
- in a still solution, concentration builds at the solute surface causing higher crystallization, therefore, stirring the solution prevents the build up which will maximize the net dissolving rate.
- net dissolving rate is the rate of dissolving rate minus the crystallization rate.
- when the rate of solubility and crystallization is the same, solution is saturated, and dynamic equilibrium is reached.
- Le Chatelier's Principle shows the responses of equilibrium change when equilibrium system is subjected to change in temperature, pressure or concentration. This principle states:
- With an increase of temperature, solubility increases which causes an endothermic reactions.
- With an increase of temperature, solubility decreases which causes an exothermic reactions.
- Adding an inert gas to a constant volume, equilibrium mixture has no effect on the equilibrium.
- An increase in external pressure causes a decrease in reaction volume and shifts equilibrium to the right.
- Petrucci, Harwood, Herring. General Chemistry: Principles & Modern Applications. 8th ed. Upper Saddle River, New Jersey: Pearson/Prentice Hall, 2002.
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