Lesson 13 Notes

A. Solubility:

Solubility of a substance is partly related to the affinity (any link or connection) of the solvent and solute molecules.

Solubility is also affected by several other factors, one of which is temperature. How they behave depends on whether heat is given off (exothermic) or taken in (endothermic) when they dissolve.

- Endothermic: (Sodium chloride and sodium nitrate) increase in solubility with an increase in temperature.

- Exothermic: (calcium hydroxide) increase in solubility with increasing temperature.

- example: if water is heated bubbles of gas become visible long before the water reaches its boiling point. These are dissolved gases, such as oxygen and nitrogen, coming out of the solution.

The degree of which solubility changes with temperature varies depending on the type of solute and solvent.

- Example: the solubility of sodium chloride in water increases only slightly between 20 degree C and 80 degree C; sodium nitrate approximately doubles its solubility over the same temperature range.

When an unsaturated solution of a solid is cooled, it may become saturated.

When cooling continues, some solid solute precipitates out, usually as crystals – a process known as recrystallization.

Example: the hot copper II sulfate solution you make for this lesson is unsaturated when hot but becomes saturated when cooled, with crystals appearing in the test tube.

b. Upon cooling, saturated solutions will often become supersaturated. This means they contain more dissolved solute than is present in the saturated solution.

c. The different solubilities of substances can be used to separate different solutes in a solution through a process called factional crystallization.