Question
A paper dipped in a dil. H_2SO_4 solution of 'X' upon treatment with SO_2 gas turns into green. The compound 'X' is :
JEE Main
Chemistry
d and f Block Elements
2026
JEE Main 2026 (05 April Shift 2)
JEE Main Chemistry Question (2026) — Solution
Options
- A. KI-starch
- B. KMnO_4
- C. Pb(CH_3COO)_2
- D. K_2Cr_2O_7
Answer
D. K_2Cr_2O_7
Step-by-step solution
SO_2 gas acts as a reducing agent. When a paper dipped in an acidified solution of potassium dichromate (K_2Cr_2O_7) is exposed to SO_2 gas, the orange-colored dichromate ion (Cr_2O_7^ 2- ) is reduced to the green-colored chromium(III) ion (Cr^ 3+ ). The chemical reaction is: K_2Cr_2O_7 + H_2SO_4 + 3SO_2 K_2SO_4 + Cr_2(SO_4)_3 + H_2O The formation of chromium(III) sulfate, Cr_2(SO_4)_3, is responsible for the green color. Thus, the compound 'X' is K_2Cr_2O_7. Answer: K_2Cr_2O_7
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