Hardness in water is caused primarily by the bicarbonates of calcium and magnesium.

• A. Alkalinity
• B. Hardness
• C. Dissolved Gases
• D. Pathogens

Answer: (B)

Water hardness comes from dissolved calcium and magnesium ions. In many natural waters these ions are present as bicarbonates, forming calcium bicarbonate and magnesium bicarbonate. These bicarbonates contribute to temporary hardness because heating causes them to decompose into carbonate and CO2, with calcium carbonate precipitating as scale. Alkalinity is about the water’s buffering capacity (mainly from bicarbonates, carbonates, and hydroxides) and is related but not the same as hardness. Dissolved gases and pathogens don’t define hardness. So, hardness being caused by calcium and magnesium bicarbonates aligns with the idea that calcium and magnesium ions — especially in their bicarbonate forms — are what produce hardness.