Physics and Chemistry Fundamentals› Basic Chemistry and Chemicals in Everyday Life
Permanent hardness of water is due to the presence of :
Answer a.
The answer is (a) calcium sulphate. Sulphates (and chlorides) of calcium and magnesium stay dissolved on boiling, so they cause permanent hardness. Hard water contains dissolved calcium and magnesium salts, which form a scum with soap. Temporary hardness comes from their bicarbonates, which boiling turns into insoluble calcium carbonate or magnesium hydroxide: Ca(HCO3)2 gives CaCO3 + H2O + CO2, and Mg(HCO3)2 gives Mg(OH)2 + 2CO2. Permanent hardness comes from the chlorides and sulphates of calcium and magnesium. These do not break down on boiling, so boiling cannot remove them. Calcium sulphate is the one such salt among the options.
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