Solid Solubility and Temperature
The solubility of a given solute in a given solvent typically depends on temperature. For many solids dissolved in liquid water, the solubility increases with temperature up to 100 °C. In liquid water at high temperatures (e.g., that approaching the critical temperature), the solubility of ionic solutes tends to decrease due to the change of properties and structure of liquid water; the lower dielectric constant results in a less polar solvent.
The chart (Figure 1) shows solubility curves for some typical solid inorganic salts. Many salts behave like barium nitrate and disodium hydrogen arsenate, and show a large increase in solubility with temperature. Some solutes (e.g., sodium chloride in water) exhibit solubility that is fairly independent of temperature. A few, such as cerium(III) sulfate, become less soluble in water as temperature increases. This temperature dependence is sometimes referred to as retrograde or inverse solubility, and exists when a salt's dissolution is exothermic (according to Le Chatelier's principle, extra heat will cause equilibrium for an exothermic process to shift towards the reactants). Occasionally, a more complex pattern is observed, as with sodium sulfate, where the less soluble decahydrate crystal loses water of crystallization at 32 °C to form a more soluble anhydrous phase. The following figure shows solubility curves for some typical solid inorganic salts: Figure 1
As the temperature of a solution is increased, the average kinetic energy of the molecules that make up the solution also increases. This increase in kinetic energy allows the solvent molecules to more effectively break apart the solute molecules that are held together by intermolecular attractions.
The average kinetic energy of the solute molecules also increases, destabilizing the solid state. The increased vibration (kinetic energy) of the molecules causes them to be less able to hold together, and thus they dissolve more readily.
A useful heuristic is to consider that as the temperature of a solid is increased, it is closer to its melting point, and thus closer to a liquid, and it is easier to dissolve something that is closer to a liquid.
APPLICATION IN RECRYSTALLIZATION
One application of the differential solubility of compounds at different temperatures is to perform a recrystallization. An impure substance is taken up in a volume of solvent at a temperature at which it is insoluble, but is heated until it becomes soluble. The impurities dissolve as well, but when the solution is cooled it is often possible to selectively crystallize (precipitate) the desired substance in a purer form.