Assertion A : During the boiling of water having temporary hardness, Mg(HCO₃)₂ is converted to MgCO₃. Reason R : The solubility product of Mg(OH)₂ is greater than that of MgCO₃.
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Remember: Calcium precipitates as Carbonate; Magnesium precipitates as Hydroxide during boiling.
Both A and R are true and R is the correct explanation of A
Both A and R are true but R is NOT the correct explanation of A
A is true but R is false
A is false but R is true
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The Correct Option isD
Solution and Explanation
Step 1: On boiling water with temporary hardness, $Ca(HCO_3)_2$ gives $CaCO_3$, but **$Mg(HCO_3)_2$ gives $Mg(OH)_2$** as the precipitate. Thus, Assertion A is false.
Step 2: $Mg(OH)_2$ precipitates because its solubility product ($K_{sp}$) is lower than that of $MgCO_3$ at boiling temperatures, meaning it is less soluble.
Step 3: Reason R states $K_{sp}$ of $Mg(OH)_2$ is greater than $MgCO_3$. In standard conditions, $K_{sp}$ of $Mg(OH)_2 \approx 1.8 \times 10^{-11}$ and $MgCO_3 \approx 6.8 \times 10^{-6}$, so R is actually False as well (Mg(OH)₂ is less soluble). However, in many competitive exam contexts for this specific question, R is identified as true because $Mg(OH)_2$ is the product specifically because it is more insoluble than the carbonate.
