JEE Main 2021 · 26 August · evening · Single correct · +4 / −1
Given below are two statements : one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A) : Barium carbonate is insoluble in water and is highly stable.
Reason (R) : The thermal stability of the carbonates increases with increasing cationic size.
- (a) Both (A) and (R) are true and (R) is the true explanation of (A)
- (b) (A) is true but (R) is false
- (c) Both (A) and (R) are true but (R) is not the true explanation of (A)
- (d) (A) is false but (R) is true.
Answer and solution
Answer (a) Both (A) and (R) are true and (R) is the true explanation of (A)
Solution
Chemistry ma’am · 4 steps
The bigger the cation, the less it pulls on the carbonate ion, so BaCO₃ is the hardest to break.
- Check the Assertion. Alkaline earth carbonates are only sparingly soluble in water, and BaCO₃ is practically insoluble. It also decomposes only at a very high temperature, so (A) is true.
- Why size matters. On heating, MCO₃ → MO + CO₂. A small cation with a high charge density polarises the large CO₃²⁻ ion and weakens its C–O bonds, so the carbonate breaks up easily. A large cation like Ba²⁺ hardly distorts the carbonate ion, so the carbonate stays intact.
Carbonate Cation radius Decomposes near MgCO₃ 72 pm 540 °C CaCO₃ 100 pm 900 °C SrCO₃ 118 pm 1290 °C BaCO₃ 135 pm, largest 1360 °C, most stable - Link A and R. Thermal stability rises down the group as the cation grows, so (R) is true. Ba²⁺ is the largest cation here, and that is exactly why BaCO₃ is so stable.
- So. Both statements are true and (R) explains (A), so the answer is (a).
The idea to keep. For carbonates, nitrates and hydroxides of the s-block, think polarising power: a smaller cation distorts the big anion more, so the compound decomposes more easily. Stability rises down the group.
Answer from the NTA final answer key. Solution by PrepWiser.