A paper dipped in a dil. H2SO4 solution of 'X' upon treatment with SO2 gas turns into green. The compound 'X' is :
- KI-starch
- KMnO4
- Pb(CH3COO)2
- K2Cr2O7
Answer
(4) K2Cr2O7
Solution
SO2 is a reducing agent. Acidified potassium dichromate (orange) is reduced to Cr3+ (green):
Cr2O72- + 3SO2 + 2H+ 2Cr3+ + 3SO42- + H2O
⇒ the paper turns from orange to green.
Other options: KI-starch paper turns blue with oxidising agents; acidified KMnO4 is decolourised (not green) by SO2; lead acetate paper turns black with H2S.
∴ X = K2Cr2O7
Answer: (4)