What substance may be added to municipal drinking water to make it clearer?

• A. Chlorine
• B. Fluoride
• C. Alum
• D. Iodine

Answer: (C)

Alum works as a coagulant in water treatment. Tiny, negatively charged particles and dissolved irregularities in water don’t easily settle and give water its cloudy appearance. When alum is added, it helps form aluminum hydroxide, which neutralizes those negative charges and causes the small particles to stick together into larger clumps, or flocs. These flocs are heavy enough to settle out or be filtered away, so the water becomes clearer. Chlorine, fluoride, and iodine serve other purposes in water treatment—chlorine disinfects, fluoride helps with dental health, and iodine isn’t used for clarification.