Hardness in water is primarily due to which ions?

• A. Calcium and magnesium.
• B. Sodium and potassium.
• C. Nitrate and phosphate.
• D. Chloride and sulfate.

Answer: (A)

Hardness in water comes from dissolved calcium and magnesium ions. These multivalent cations come from minerals in rocks and soils and readily form compounds that cause scale and soap scum, giving water its “hard” character. The other ions listed don’t drive hardness: sodium and potassium are monovalent and don’t contribute much to hardness; nitrate and phosphate are anions related to nutrient pollution; chloride and sulfate are salts that don’t determine hardness in the same way. In fact, hardness can be temporary when calcium and magnesium are present as bicarbonates (boiling can reduce this), while sulfates and chlorides of calcium and magnesium can contribute to permanent hardness. So the key contributors are calcium and magnesium.