Q5 / 180 · COMEDK 2021 · Chemistry
An aqueous solution of $X$ on addition of hydrogen peroxide in ice cold conditions gives blue colour to the ethereal layer. Then, $X$ can be
- Correct answerAacidified potassium chromate
- Balkaline potassium dichromate
- Cacidified potassium manganate
- Dacidified potassium permanganate
Solution
The formation of a blue colour in the ethereal layer upon the addition of hydrogen peroxide ($H_{2}O_{2}$) to an aqueous solution in ice-cold conditions is a characteristic test for the presence of chromium(VI).
When $H_{2}O_{2}$ reacts with acidified dichromate or chromate solutions, it forms chromium pentoxide ($CrO_{5}$), which is blue in colour. The reaction is as follows:
$Cr_{2}O_{7}^{2-} + 2H^{+} + 4H_{2}O_{2} \rightarrow 2CrO_{5} + 5H_{2}O$
The $CrO_{5}$ species is unstable in aqueous solution and decomposes rapidly, but it is stabilized in ether, where it imparts a deep blue colour to the ethereal layer. Among the given options, acidified potassium chromate contains the chromate ion ($CrO_{4}^{2-}$), which in acidic medium exists in equilibrium with the dichromate ion ($Cr_{2}O_{7}^{2-}$), thus satisfying the condition for the test.
Answer: acidified potassium chromate
When $H_{2}O_{2}$ reacts with acidified dichromate or chromate solutions, it forms chromium pentoxide ($CrO_{5}$), which is blue in colour. The reaction is as follows:
$Cr_{2}O_{7}^{2-} + 2H^{+} + 4H_{2}O_{2} \rightarrow 2CrO_{5} + 5H_{2}O$
The $CrO_{5}$ species is unstable in aqueous solution and decomposes rapidly, but it is stabilized in ether, where it imparts a deep blue colour to the ethereal layer. Among the given options, acidified potassium chromate contains the chromate ion ($CrO_{4}^{2-}$), which in acidic medium exists in equilibrium with the dichromate ion ($Cr_{2}O_{7}^{2-}$), thus satisfying the condition for the test.
Answer: acidified potassium chromate
Hence, the correct answer is option: 'A'.