Hardness is generally expressed as the total concentration of which ions?

• A. Sodium and Chloride ions
• B. Hydrogen and Hydroxide ions
• C. Calcium and Magnesium ions
• D. Carbonate and Bicarbonate

Answer: (C)

Hardness measures the presence of dissolved multivalent cations, mainly calcium and magnesium. These ions come from minerals in water sources and are the ones that form scale and soap scum, which is why their concentration determines how hard the water feels and behaves. Hardness is typically expressed as milligrams per liter of calcium carbonate equivalents, reflecting the total amount of Ca2+ and Mg2+ present.

Sodium and chloride affect salinity and taste more than hardness. Hydrogen and hydroxide relate to pH and basicity, not the total hardness. Carbonate and bicarbonate relate to buffering and can influence temporary hardness when paired with Ca2+ or Mg2+, but they themselves are not the ions used to express hardness.