JEE Main 2023 · 29 January · evening · Single correct · +4 / −1
A solution of Cr O₅ in amyl alcohol has a __________ colour.
- (a) Yellow
- (b) Orange-Red
- (c) Green
- (d) Blue
Answer and solution
Answer (d) Blue
Solution
Chemistry ma’am · 3 steps
This is the classic dichromate test: acidified dichromate plus H₂O₂ gives deep blue CrO₅.
- Where CrO5 comes from. Add H₂O₂ to an acidified dichromate solution and CrO₅ (chromium peroxide) forms. It is unstable in water, so it is shaken into an organic layer such as amyl alcohol or ether. There it stays and shows a deep blue colour.
Cr₂O₇²⁻ + 2H⁺ + 4H₂O₂ → 2CrO₅ + 5H₂O
- Its structure. CrO₅ is CrO(O₂)₂, with one oxo oxygen and two peroxo groups in a butterfly shape. So Cr is +6 (-2 from the oxo group, -4 from the two peroxo groups), not +10. The blue colour comes from charge transfer, because Cr⁶⁺ has no d electrons for d–d transitions.
Chromium species Colour CrO₄²⁻ (chromate) yellow Cr₂O₇²⁻ (dichromate) orange-red Cr³⁺ green CrO₅ in amyl alcohol or ether deep blue - So. The organic layer containing CrO₅ is blue, which is option (d).
The idea to keep. Learn the chromium colour ladder together: chromate yellow, dichromate orange, Cr³⁺ green, and CrO₅ blue in an organic layer. A blue organic layer is the test for chromium(VI).
Answer from the NTA final answer key. Solution by PrepWiser.