Saturated Solutions and Solubility

• When a salt (NaCl) crystal is initially place in a sample of H₂O the solution is devoid of hydrated Na⁺ and Cl^- ions:

• As the water molecules surround, separate and disperse the Na⁺ and Cl^- ions, the solution becomes populated by the hydrated ions:

• The dispersed ions in the solution will collide with water molecules, the surfaces of the container and potentially other ions as well.
• If the original crystal has not completely dissolved, then dispersed ions can also collide with the remaining crystal. These collisions can result in the incorporation of the ions back into the NaCl crystal lattice:

• Thus, there are two opposing processes that can potentially occur:

1. Dissolving of the crystal, resulting in hydration of the individual Na⁺ and Cl^- ions, and
2. Collision of dispersed ions resulting in an increase in the crystal mass (a process also known as Crystallization)

These two opposing processes of dissolving and crystallization can be represented as follows:

• If the rate of dissolution is greater than the rate of crystallization, then the crystals of NaCl in the solvent will get smaller
• If the rate of crystallization is greater than the rate of dissolution, then the crystals of NaCl in the solvent will get larger

• If the rates of the two opposing processes are equal, then the size of the NaCl crystals will remain unchanged and the system is said to be in a dynamic equilibrium

A solution that is in dynamic equilibrium with undissolved solute is said to be saturated (i.e. no more solute will dissolve into the solvent under the current conditions)

• The concentration of solute present in the solution under conditions of saturation is known as the solubility of that solute

• For example, at 0°C 35.7g of NaCl can be dissolved in a total volume of 100ml of H₂O. This is the solubility of NaCl in H₂0 at 0°C.
• At higher temperatures, usually more solute can be dissolved, and the solubility is higher.

Under some conditions it is possible to produce a supersaturated solution of a solute

• Solute is dissolved to saturation at a high temperature
• The solution is carefully and slowly cooled to a lower temperature (the idea is not to induce the formation of tiny crystals that can serve to nucleate crystal growth)
• At the lower temperature, the concentration of solute is higher than the equilibrium concentration at that temperature. The introduction of a "seed" crystal will stimulate rapid crystal formation