Question

When orange solution containing $C{r}_{2}O{{}_7}^{2-}$ ion is treated with an alkali, a yellow solution is formed and when $H^{+}$ ions are added to yellow solution, an orange solution is obtained. Explain why does this happens?

Answer 1

When dichromate ion $C{r}_2{O}_7^{2-}$ is treated with an alkali, the orange colour gets changed into yellow due to the formation of chromate ion $Cr{O}_4^{2-}$

$C{r}_2{O}_7^{2-}+2O{H}^{-}\to 2Cr{O}_4^{2-}+H_{2}O$ (Orange) (Yellow)

When an acid solution or $H^{+}$ ions are added to the yellow solution, the yellow colour of chromate ion changes into orange due to the formation of dichromate ion.

$2Cr{O}_4^{2-}+2H^{+}\to C{r}_2{O}_7^{2-}+H_{2}O$ (Yellow) (Orange)