Solubility and Temperature
The concentration at which no more solute dissolves in a solution for a given temperature and pressure is the saturation point. The molarity of a compound in a saturated solution is the molar solubility. An unsaturated solution is one in which more solute can be dissolved into the solution. If more solute is added to a saturated solution, the solute stays in its undissolved form.
A solution's solubility is a function of temperature. For a solid dissolved in a liquid, solubility usually increases with increasing temperature. More of the solid will dissolve in a warm solvent than in a cold one. This property can be used to produce a solution that is supersaturated, which means the solution contains more solute than can be dissolved in the solution under normal circumstances. First, a saturated solution of concentration Chot is made at a high temperature. If the resulting solution is allowed to cool slowly, the solvent can stay in solution even though the solute concentration Chot is greater than the solubility at the lower temperature, Ccool. Such a solution is unstable because the solvent particles still surround the solute particles, even though at the lower temperature the attractive forces between solute particles are stronger than the attractive forces between solvent and solute particles. Crystallization of the solute forms quickly if the solution is disturbed, for example, with the introduction of a solute crystal or even by a scratch on the wall of the container, because the solute particles are close enough to overcome the attraction of the solvent particles.
For gas solutions in a liquid solvent, solubility usually increases with decreasing temperature. The gas molecules are already dissociated and spread apart from one another, so they must be brought closer together in solution. Thus, the enthalpy change in each step is:
1. Gas molecules move together: .
2. Solvent molecules spread apart: .
3. Solvent-gas molecules move together: .
For this kind of solution, is the sum of two exothermic processes and one endothermic process, and .If the temperature of the solution is raised, the gas molecules become more energetic and tend to spread apart. This is why adding heat to a solution causes the gas molecules to bubble up and leave the solution (as in a pot of boiling water). The gas solubility decreases with increasing temperature.