Hardness of water is measured in parts per million (ppm) of which two ions?

• A. Sodium and Potassium
• B. Calcium and Magnesium
• C. Chloride and Sulfate
• D. Iron and Manganese

Answer: (B)

Hardness is determined by the presence of multivalent cations in water, specifically calcium and magnesium ions. These ions form the mineral scale that clogs boilers and heat exchangers, so the concentration of Ca2+ and Mg2+ sets how hard the water is. In practice, hardness is expressed as parts per million as calcium carbonate (ppm as CaCO3), which means the amounts of calcium and magnesium are converted to an equivalent amount of CaCO3 for reporting. Other ions like sodium and potassium don’t form the same scale-building compounds, while chloride, sulfate, iron, and manganese affect other water quality aspects but not hardness itself. Thus, calcium and magnesium are the ions that define water hardness.