Hardness in water is mainly due to presence of which compounds?

• A. Calcium and Magnesium compounds
• B. Sodium chloride
• C. Iron oxide
• D. Sulfates

Answer: (A)

Hardness in water comes from dissolved calcium and magnesium ions. These two divalent cations occur as compounds of calcium and magnesium released from rocks and minerals, and they stay dissolved in the water, giving it the ability to form scale and to react with soaps. When carbonate hardness is involved, bicarbonates of calcium and magnesium can precipitate as calcium carbonate on heating, which is why hard water often forms scaly deposits. While sulfates can be part of salts that include calcium or magnesium, and thus contribute to hardness, the primary source of hardness is the presence of calcium and magnesium compounds. Sodium chloride is not a source of hardness, and iron oxide is a solid contaminant that doesn’t cause dissolved hardness.