2020 · Q11
Which one of the following methods can be used to remove permanent hardness of water?
general-sciencechemistry
- ABoiling
- BIon-exchange
- CLime treatment
- DBubbling sulphur dioxide gas
Explanation
Permanent hardness is caused by dissolved calcium and magnesium chlorides and sulfates, which boiling cannot remove — boiling only drives off temporary (bicarbonate) hardness. Ion-exchange resins replace Ca²⁺ and Mg²⁺ with Na⁺, removing permanent hardness. Lime treatment (Clark's process) targets temporary hardness, and bubbling sulfur dioxide does not soften water.