Precipitation

The steps in the water-treatment process described above are used to remove insoluble particles from the water supply. But recall from before ("Species (Other Than H₂O) Contained in Water") that water also contains many molecules and ions in solution. Many of the ions in solution can be removed by precipitation: reacting the ions (to be removed) with other ions to produce insoluble solids that can be removed by sedimentation (see above) or filtration (see below).

A typical precipitation reaction used to remove ions in water treatment follows the reaction shown in Equation 3, below. This is the same reaction type that you performed in the Experiment when the reaction between ions from two aqueous solutions produced a solid precipitate.

Two major classes of ions are typically removed via precipitation:

• Calcium (Ca²⁺) ions and magnesium (Mg²⁺) ions that have been leached from minerals in the ground cause the condition known as "water hardness". These ions do not pose any health threat, but they can engage in reactions that leave insoluble mineral deposits, such as scum rings on bathtubs and cooking vessels, or scale on industrial boilers, which decreases the boilers' efficiency. These deposits can make hard water unsuitable for many uses.
• Iron (Fe²⁺) ions and manganese (Mn²⁺) ions can stain plumbing fixtures and laundered clothes. These ions may also promote the growth of certain bacteria, which give foul tastes and odors to the water.