JEE Main 2022 · 26 June · evening · Single correct · +4 / −1
Boiling of hard water is helpful in removing the temporary hardness by converting calcium hydrogen carbonate and magnesium hydrogen carbonate to :
- (a) CaCO₃ and Mg(OH)₂
- (b) CaCO₃ and MgCO₃
- (c) Ca(OH)₂ and MgCO₃
- (d) Ca(OH)₂ and Mg(OH)₂
Answer and solution
Answer (a) CaCO₃ and Mg(OH)₂
Solution
Chemistry ma’am · 3 steps
Calcium stops at the carbonate, but magnesium goes one step further to the less soluble hydroxide.
- Calcium on boiling. Heat breaks the hydrogen carbonate down: Ca(HCO₃)₂ →(Δ) CaCO₃ v + H₂O + CO₂ ↑. Insoluble CaCO₃ settles out and can be filtered off.
- Magnesium on boiling. MgCO₃ forms first, but it is fairly soluble. In hot water it hydrolyses to Mg(OH)₂, which has a much lower solubility product and so precipitates out.
Mg(HCO₃)₂ →(Δ) Mg(OH)₂ v + 2CO₂ ↑
- So. Boiling converts the two hydrogen carbonates into CaCO₃ and Mg(OH)₂, which is option (a).
The idea to keep. Removing hardness means forming the least soluble compound. For calcium that is CaCO₃. For magnesium it is Mg(OH)₂, not MgCO₃.
Answer from the NTA final answer key. Solution by PrepWiser.