JEE Main 2020 · 5 September · evening · Single correct · +4 / −1
The one that is NOT suitable for the removal of permanent hardness of water is :
- (a) Ion-exchange method
- (b) Calgon's method
- (c) Treatment with sodium carbonate
- (d) Clark's method
Answer and solution
Answer (d) Clark's method
Solution
Chemistry ma’am · 3 steps
Ask each method one thing: can it remove CaCl₂ and CaSO₄, not just bicarbonates?
- Two kinds of hardness. Temporary hardness comes from Ca(HCO₃)₂ and Mg(HCO₃)₂. Permanent hardness comes from chlorides and sulphates such as CaCl₂, CaSO₄ and MgSO₄, which boiling cannot remove.
- Clark's method. Clark's method adds a calculated amount of slaked lime, Ca(OH)₂. Lime reacts only with the bicarbonate ion, turning it into insoluble CaCO₃.
Ca(HCO₃)₂ + Ca(OH)₂ → 2CaCO₃ v + 2H₂O
Method How it works Removes Cl⁻ / SO₄²⁻ hardness? Ion exchange Resin or zeolite swaps Ca²⁺, Mg²⁺ for Na⁺ or H⁺ Yes Calgon Sodium hexametaphosphate holds Ca²⁺, Mg²⁺ in a soluble complex Yes Washing soda CaCl₂ + Na₂CO₃ → CaCO₃ v + 2NaCl Yes Clark's Lime reacts only with HCO₃⁻ No, temporary hardness only - So. Clark's method removes only temporary hardness and cannot remove permanent hardness. The answer is (d).
The idea to keep. Boiling and Clark's lime work only on bicarbonates, so they remove temporary hardness. Washing soda, Calgon and ion exchange take out the Ca²⁺ and Mg²⁺ ions themselves, so they remove both kinds.
Answer from the NTA final answer key. Solution by PrepWiser.