JEE Main · 2026 · 05 Apr 2026 · Shift 2 · d and f Block Elements

A paper dipped in a dil. H₂SO₄ solution of 'X' upon treatment with SO₂ gas turns into green. The compound 'X' is :

Answer: (D)

Solution

Sulphur dioxide acts as a reducing agent in acidic solution. Potassium dichromate, which is orange, is reduced by

S O_{2}

in dilute sulphuric acid to chromium(III) sulphate, and the

C r^{3 +}

ion gives the characteristic green colour:

K_{2} C r_{2} O_{7} + 3S O_{2} + H_{2} S O_{4} \to C r_{2} (S O_{4})_{3} + K_{2} S O_{4} + H_{2} O

. The compound X that makes the paper turn green with

S O_{2}

K_{2} C r_{2} O_{7}

JEE Main · 2026 · 05 Apr 2026 · Shift 2 · Single correct option