Which ions are responsible for water hardness and potential scaling?

• A. Sodium and Potassium
• B. Chloride and Nitrate
• C. Sulfate and Phosphate
• D. Calcium and Magnesium

Answer: (D)

Water hardness and potential scaling come from dissolved calcium and magnesium ions. These multivalent cations are the main contributors to hardness because they readily form compounds with carbonate and sulfate in water, which precipitate as scale on pipes, boilers, and other surfaces. When water contains bicarbonates of calcium and magnesium, heating or changing conditions can drive formation of calcium carbonate and magnesium carbonate, causing temporary hardness to turn into scale as the carbonates precipitate. The other ions listed don’t drive hardness in the same way: sodium and potassium are monovalent and don’t produce the stubborn carbonate scales; chloride and nitrate are anions that don’t contribute to hardness; sulfates and phosphates can be involved in some scaling scenarios but are not the primary hardness-causing ions. Thus, calcium and magnesium are responsible for water hardness and scaling.