Which one among the following salts causes hardness to the water?

[amp_mcq option1=”Calcium bicarbonate” option2=”Calcium carbonate” option3=”Sodium sulphate” option4=”Potassium chloride” correct=”option1″]

This question was previously asked in

UPSC CAPF – 2012

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Water hardness is caused by dissolved multivalent cations, primarily calcium (Ca²⁺) and magnesium (Mg²⁺) ions. These ions are usually present in water as salts like bicarbonates, sulfates, or chlorides.

Calcium bicarbonate (Ca(HCO3)₂) is a common salt found dissolved in water that causes temporary hardness. When heated, calcium bicarbonate decomposes to form calcium carbonate, which precipitates out, reducing the hardness. Calcium carbonate (CaCO₃) itself is largely insoluble in water under normal conditions and does not dissolve significantly to cause hardness.

Sodium salts (like sodium sulphate) and potassium salts (like potassium chloride) do not cause water hardness, as sodium (Na⁺) and potassium (K⁺) are monovalent ions. The term “hardness” specifically refers to the presence of divalent or trivalent metallic cations.

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