JEE Main 2021 · 25 July · morning · Single correct · +4 / −1
Given below are two statements :
Statement I : None of the alkaline earth metal hydroxides dissolve in alkali.
Statement II : Solubility of alkaline earth metal hydroxides in water increases down the group.
In the light of the above statements, choose the most appropriate answer from the options given below :
- (a) Statement I is correct but Statement II is incorrect.
- (b) Statement I is incorrect but Statement II is correct.
- (c) Statement I and Statement II both are incorrect.
- (d) Statement I and Statement II both are correct.
Answer and solution
Answer (b) Statement I is incorrect but Statement II is correct.
Solution
Chemistry ma’am · 4 steps
Remember beryllium is the odd one out: its hydroxide is amphoteric, so it does dissolve in alkali.
- Check Statement I. Most alkaline earth hydroxides are basic, but Be(OH)₂ is amphoteric. It reacts with acids and also with alkalis such as NaOH, forming the soluble beryllate ion.
Be(OH)₂ + 2OH⁻ → [Be(OH)₄]²⁻
- Statement I fails. One hydroxide dissolving in alkali is enough to make "none of them dissolve" wrong. So Statement I is incorrect.
- Check Statement II. Going down the group from Be(OH)₂ to Ba(OH)₂, the cation gets bigger and lattice enthalpy falls faster than hydration enthalpy. So the hydroxides dissolve more easily in water, and Statement II is correct.
- So. Statement I is incorrect and Statement II is correct, which is option (b).
The idea to keep. Beryllium behaves like aluminium: its oxide and hydroxide are amphoteric. For Group 2, hydroxide solubility rises down the group, while sulphate solubility falls.
Answer from the NTA final answer key. Solution by PrepWiser.