$\mathrm{K_2Cr_2O_7}$ is a powerful oxidising agent. In dilute sulphuric acidmedium the oxidation state of Cr changes from +6 to +3. The oxidisingaction can be represented as follows: $\mathrm{Cr_2O_7^{2-}} + 14\mathrm{H}^{+} + 6e^{-} \rightarrow 2\mathrm{Cr}^{3+} + 7\mathrm{H_2O}$ (i) Iodide : Iodide ion $(\mathrm{I}^{-})$ is oxidised to $\mathrm{I_2}$ by the acidfied solution of$\mathrm{K_2Cr_2O_7}$. Reaction : (ii) Iron(II) solution : Ferrous salts $(\mathrm{Fe}^{2+})$ are oxidised to ferric $(\mathrm{Fe}^{3+})$saltswhen they are treated with acidified $\mathrm{K_2Cr_2O_7}$.Reaction :$\text{ (iii) } \mathrm{H_2S}$ : $\mathrm{H_2S}$ is oxidised to sulphur.