Concentrations of hardness are expressed in terms of mg/L as which compound?

• A. Calcium Carbonate
• B. Magnesium hydroxide
• C. Sodium sulfate
• D. Calcium sulfate

Answer: (A)

Hardness is a measure of dissolved calcium and magnesium ions, and expressing it as mg/L as CaCO3 provides a single, standard reference to compare different waters. Calcium carbonate is used because its chemical behavior is well-defined, and concentrations of Ca2+ and Mg2+ can be converted to an equivalent amount of CaCO3. This makes it easy to express the total hardness with one value and to relate it to treatment steps.

If a sample has a certain amount of Ca2+ and Mg2+, you convert those ions to their CaCO3 equivalents and report the result as mg/L of CaCO3. The other substances listed (magnesium hydroxide, sodium sulfate, calcium sulfate) may contribute to hardness, but they are not the standard reference compound used to express hardness, which is why calcium carbonate is the correct basis.