Which statement about carbonate hardness is correct?

Which statement about carbonate hardness is correct?

• A. Caused by dissolved carbon dioxide alone
• B. Is equal to noncarbonate hardness
• C. Alkalinity present in water up to the total hardness
• D. Is solely determined by magnesium ions

Answer: (C)

Carbonate hardness comes from calcium and magnesium in the form of carbonates and bicarbonates, and it sits at the heart of the water’s buffering system. Alkalinity is the water’s ability to neutralize acids, largely due to those same carbonate and bicarbonate species. Total hardness equals the sum of carbonate hardness and noncarbonate hardness, so the alkalinity that arises from carbonate chemistry cannot exceed the total hardness. In water where all hardness is carbonate-based, alkalinity and carbonate hardness are essentially the same. That’s why the statement that alkalinity is present in water up to the total hardness is correct: alkalinity is a measure tied to the carbonate portion of hardness, and it is bounded by the overall hardness. The other options are not accurate because carbonate hardness is not caused by dissolved carbon dioxide alone, it is not equal to noncarbonate hardness, and it involves both calcium and magnesium, not just magnesium.

Carbonate hardness comes from calcium and magnesium in the form of carbonates and bicarbonates, and it sits at the heart of the water’s buffering system. Alkalinity is the water’s ability to neutralize acids, largely due to those same carbonate and bicarbonate species. Total hardness equals the sum of carbonate hardness and noncarbonate hardness, so the alkalinity that arises from carbonate chemistry cannot exceed the total hardness. In water where all hardness is carbonate-based, alkalinity and carbonate hardness are essentially the same. That’s why the statement that alkalinity is present in water up to the total hardness is correct: alkalinity is a measure tied to the carbonate portion of hardness, and it is bounded by the overall hardness. The other options are not accurate because carbonate hardness is not caused by dissolved carbon dioxide alone, it is not equal to noncarbonate hardness, and it involves both calcium and magnesium, not just magnesium.