Hardness is caused by polyvalent metallic cations such as

Hardness is caused by polyvalent metallic cations such as

• A. calcium and magnesium
• B. sodium and potassium
• C. iron and copper
• D. chloride and sulfate

Answer: (A)

Hardness in water comes from polyvalent metallic cations—ions with more than one positive charge. The classic culprits are calcium and magnesium, both carrying a 2+ charge (Ca2+ and Mg2+) and typically derived from minerals like limestone and dolomite that dissolve into the water. Because these ions have two positive charges, they readily form salts with carbonate, sulfate, and other anions, which leads to scale formation and soap not lathering well. Sodium and potassium are monovalent cations, so they don’t contribute to hardness in the same way. Chloride and sulfate are anions, not cations, so they aren’t hardness culprits. Iron and copper are multivalent cations too, but the primary, most common source of water hardness is calcium and magnesium.

Hardness in water comes from polyvalent metallic cations—ions with more than one positive charge. The classic culprits are calcium and magnesium, both carrying a 2+ charge (Ca2+ and Mg2+) and typically derived from minerals like limestone and dolomite that dissolve into the water. Because these ions have two positive charges, they readily form salts with carbonate, sulfate, and other anions, which leads to scale formation and soap not lathering well. Sodium and potassium are monovalent cations, so they don’t contribute to hardness in the same way. Chloride and sulfate are anions, not cations, so they aren’t hardness culprits. Iron and copper are multivalent cations too, but the primary, most common source of water hardness is calcium and magnesium.