SOLUBILITY PRODUCT CONSTANTS
Like the acids and bases discussed in the previous section, another group of substances comprised generally of ionic compounds that have very limited solubility in water deserves separate treatment. Examples include magnesium hydroxide (commonly known as milk of magnesia, an antacid), barium sulfate, (used in enemas to diagnose colonic tumors), and silver chloride. These substances are generally referred to as “insoluble” or “slightly soluble,” and are governed by a solubility equilibrium expression and a solubility product constant known as Ksp, identical in concept to the principle of equilibrium discussed in Section 2.3.
The solubility of any salt depends on temperature. The solubility of most salts increases with increasing temperature. When a salt has dissolved to the maximum extent in a given volume of water at a given temperature, the solution is said to be saturated. Until that point is reached, the solution is unsaturated. If a saturated solution is heated above room temperature, more salt can be dissolved and be held in solution. If this solution is later slowly cooled (sometimes called reversibly cooled), it is possible to cool it back down to room temperature (or below), such that all of the dissolved salt can exist in the solution phase without precipitating out. Once this lower temperature is reached, the solution is said to be supersaturated. A sudden shock, for example, a mechanical disturbance, however, can cause instantaneous precipitation and the corresponding release of energy in the form of heat. The solubility of a salt is commonly measured in units of moles per liter, grams per liter, or milligrams per liter (often referred to as parts per million or simply ppm).