Which ions are responsible for water hardness?

• A. Calcium and Magnesium
• B. Sodium and Potassium
• C. Chloride and Sulfate
• D. Iron and Manganese

Answer: (A)

Water hardness comes from dissolved calcium and magnesium ions. These divalent cations carry a 2+ charge, so they readily form salts with carbonate, sulfate, and other anions in water. Their presence causes two practical effects: they react with soap to form scum instead of lather, and they can precipitate as scale (like calcium carbonate) when water is heated. That’s why calcium and magnesium are the ions responsible for hardness.

Sodium and potassium are monovalent and don’t contribute to hardness in the same way. Chloride and sulfate are anions and, by themselves, don’t cause hardness (they may accompany Ca2+ or Mg2+ in salts, but the hard-water effect comes from the calcium and magnesium ions). Iron and manganese can cause discoloration and taste issues but aren’t the primary contributors to hardness.