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JEE Main20265 April 2026Evening ShiftChemistryd and f Block ElementsActual

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

Options

  1. AKI-starch
  2. BKMnO₄
  3. CPb(CH₃COO)₂
  4. DK₂Cr₂O₇

Correct answer

D. K₂Cr₂O₇

Step-by-step solution

SO₂ gas acts as a reducing agent. When a paper dipped in an acidified solution of potassium dichromate ( K₂Cr₂O₇ ) is exposed to SO₂ gas, the orange-colored dichromate ion ( Cr₂O₇²⁻ ) is reduced to the green-colored chromium(III) ion ( Cr³⁺ ). The chemical reaction is: K₂Cr₂O₇ + H₂SO₄ + 3SO₂ K₂SO₄ + Cr₂(SO₄)₃ + H₂O The formation of chromium(III) sulfate, Cr₂(SO₄)₃ , is responsible for the green color. Thus, the compound ' X ' is K₂Cr₂O₇ . Answer: K₂Cr₂O₇

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