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