Explain how precipitation is used to remove pollutants from water, and give an example.

• A. Precipitation converts dissolved substances to solids that can be settled or filtered out; examples include lime softening and metal hydroxide precipitation.
• B. Precipitation increases water salinity.
• C. Precipitation refers to biological decontamination.
• D. Precipitation involves boiling water to remove solids.

Answer: (A)

Precipitation in water treatment is a process where dissolved pollutants are converted into solid particles that can be removed by settling or filtration. This happens when reagents raise the water’s pH or supply ions that form insoluble compounds with the dissolved ions, producing solid hydroxides or carbonates.

A common example is lime softening. Adding calcium hydroxide raises the pH and causes hardness-causing ions like calcium and magnesium, as well as certain metals, to precipitate as solid compounds such as calcium carbonate or metal hydroxides. The resulting solids are then settled out in a sedimentation basin or filtered away, leaving behind clearer, cleaner water. Another example is precipitating metals by forming metal hydroxides; dissolved metals react with the added base to form insoluble hydroxide solids that can be removed.

This approach is different from processes that change salinity, involve biological decontamination, or rely on boiling to remove solids.