Question

Why does sulphur dioxide turns acidified potassium dichromate paper green

Answer 1

The chemical reaction between sulphur dioxide and acidified pottasium dichromate is as follows
3SO2+ K2Cr2O7 +H2SO4→K2SO4 + Cr2(SO4)3 + H2O
In this reaction oxidation state of Cr in K2Cr2O7 is +4(Cr4+).As the reaction occur in a acidic medium Cr2(SO4)3 is formed in which the oxidation state of Cr is +3(Cr3+).​​​​​​​​​​K2Cr2O7 is orange in colour.The orange colour is changed to green because of the change in oxidation state of Cr from +4 to +3.Cr 3+ ion have green colour.