Which method is effective for removing permanent hardness from water?

• A. Boiling
• B. Distillation
• C. Filtration
• D. Ion exchange

Answer: (D)

Water hardness comes from dissolved calcium and magnesium ions in the water. Temporary hardness, caused by bicarbonates, can be removed by boiling, which converts bicarbonates to carbonates that precipitate out. But permanent hardness isn’t affected by boiling because it comes from non-bicarbonate salts of Ca2+ and Mg2+ that stay dissolved.

Ion exchange specifically targets these hardness-causing ions. As water passes through a resin loaded with sodium (or potassium) ions, the calcium and magnesium ions swap places with the sodium ions. This removes Ca2+ and Mg2+ from the water and leaves behind softened water rich in sodium ions. The resin can be regenerated with a salt solution to restore its softening ability, making it practical for continuous use.

Distillation can remove dissolved minerals but is energy-intensive and less convenient for everyday softening, and filtration only removes particles, not dissolved ions.