JEE Main 2021 · 27 July · morning · Single correct · +4 / −1
Match List - I with List - II :
Choose the most appropriate answer from the options given below
| List - I | List - II | ||
|---|---|---|---|
| (a) | NaOH | (i) | Acidic |
| (b) | Be(OH)₂ | (ii) | Basic |
| (c) | Ca(OH)₂ | (iii) | Amphoteric |
| (d) | B(OH)₃ | ||
| (e) | Al(OH)₃ |
- (a) (a)-(ii), (b)-(ii), (c)-(iii), (d)-(ii), (e)-(iii)
- (b) (a)-(ii), (b)-(iii), (c)-(ii), (d)-(i), (e)-(iii)
- (c) (a)-(ii), (b)-(ii), (c)-(iii), (d)-(i), (e)-(iii)
- (d) (a)-(ii), (b)-(i), (c)-(ii), (d)-(iii), (e)-(iii)
Answer and solution
Answer (b) (a)-(ii), (b)-(iii), (c)-(ii), (d)-(i), (e)-(iii)
Solution
Chemistry ma’am · 3 steps
Smaller, more charged central atom means less basic: Na and Ca are basic, Be and Al amphoteric, B acidic.
- The rule. A large, low-charge metal ion lets go of OH⁻ easily, so its hydroxide is basic. A small, highly charged central atom holds oxygen tightly and weakens the O–H bond instead. Its hydroxide turns amphoteric, or even acidic.
Hydroxide Central atom Behaviour Nature NaOH large Na⁺ strong alkali, releases OH⁻ basic (ii) Be(OH)₂ tiny Be²⁺ dissolves in acid to give BeCl₂, and in alkali to give [Be(OH)₄]²⁻ amphoteric (iii) Ca(OH)₂ larger Ca²⁺ lime water, a base basic (ii) B(OH)₃ boron, a non-metal boric acid, accepts OH⁻ from water: B(OH)₃ + H₂O → [B(OH)₄]⁻ + H⁺ acidic (i) Al(OH)₃ small Al³⁺ dissolves in both acid and alkali amphoteric (iii) - Diagonal link. Be is anomalous in group 2. It resembles Al, its diagonal neighbour, because the two ions have a similar charge-to-size ratio. That is why Be(OH)₂ is amphoteric like Al(OH)₃, unlike the basic Ca(OH)₂.
- So. The match is (a)-(ii), (b)-(iii), (c)-(ii), (d)-(i), (e)-(iii), which is option (b).
The idea to keep. Small, highly charged central atoms pull a hydroxide from basic towards acidic. Be and Al sit diagonally, so both are amphoteric. Boron goes all the way: B(OH)₃ is an acid.
Answer from the NTA final answer key. Solution by PrepWiser.