Solid Solubility and Temperature



 

Learning Objective

  • Recall the relationship between solubility and temperature

Key Points

    • For many solids dissolved in liquid water, the solubility increases with temperature.
    • The increase in kinetic energy that comes with higher temperatures allows the solvent molecules to more effectively break apart the solute molecules that are held together by intermolecular attractions.
    • The increased vibration (kinetic energy) of the solute molecules causes them to dissolve more readily because they are less able to hold together.

Terms

  • kinetic energyThe energy possessed by an object because of its motion, equal to one half the mass of the body times the square of its velocity.
  • solubilityThe amount of a substance that will dissolve in a given amount of a solvent to give a saturated solution under specified conditions.

Solid Solubility and Temperature

The solubility of a given solute in a given solvent typically depends on temperature. Many salts show a large increase in solubility with temperature. Some solutes 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; this can be explained because, according to Le Chatelier’s principle, extra heat will cause the equilibrium for an exothermic process to shift towards the reactants.

Solubility Versus TemperatureThis chart shows the solubility of various substances in water at a variety of temperatures (in degrees Celsius). Notice how NaCl’s solubility is relatively constant regardless of temperature, whereas Na2SO4’s solubility increases exponentially over 0–35 degrees Celsius and then abruptly begins to decrease.

Theoretical Perspective

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 with temperature, and it destabilizes the solid state. The increased vibration (kinetic energy) of the solute molecules causes them to be less able to hold together, and thus they dissolve more readily.

Application in Recrystallization

A useful application of solubility is recrystallizaton. During recrystallization, an impure substance is taken up in a volume of solvent at a temperature at which it is insoluble, which is then heated until it becomes soluble. The impurities dissolve as well, but when the solution is cooled, it is often possible to selectively crystallize, or precipitate, the desired substance in a purer form.