JEE Main 2021 · 18 March · morning · Single correct · +4 / −1
Given below are two statements : One is labelled as Assertion A and the other labelled as reason R
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.
In the light of the above statements, choose the most appropriate answer from the options given below :
- (a) Both A and R are true but R is not the correct explanation of A
- (b) A is true but R is false
- (c) Both A and R are true and R is the correct explanation of A
- (d) A is false but R is true
Answer and solution
Answer (d) A is false but R is true
Solution
Chemistry ma’am · 4 steps
Boiling hard water gives magnesium hydroxide, not magnesium carbonate. That one fact settles it.
- What boiling does. Heating breaks the hydrogencarbonate ion and drives off CO₂. With magnesium, the carbonate that would form first hydrolyses in the hot water, so the precipitate that settles out is Mg(OH)₂.
Mg(HCO₃)₂ →(boil) Mg(OH)₂ v + 2CO₂ ↑
- Check Assertion A. A says Mg(HCO₃)₂ ends up as MgCO₃. It actually ends up as Mg(OH)₂, so A is false. Calcium behaves differently: Ca(HCO₃)₂ does give CaCO₃ on boiling.
- Check Reason R. R is taken as a true statement here, but do not read it as "Mg(OH)₂ is more soluble". Mg(OH)₂ splits into three ions and MgCO₃ into two, so their Kₛₚ values have different units and cannot rank solubility directly. You also do not need R to choose: a false A already rules out (a), (b) and (c).
- So. A is false, and (d) is the only option that says so. The answer is (d): A is false but R is true.
The idea to keep. On boiling temporary hard water, calcium leaves as CaCO₃ but magnesium leaves as Mg(OH)₂. In assertion-reason questions, judge A first, because a false A often removes three options.
Answer from the NTA final answer key. Solution by PrepWiser.