When orange solution containing `Cr_(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 happen?

Explanation

On heating with alkalies, the orange colour of dichromate solution changes to yellow due to the formation of chromate ions.
`K_2Cr_2O_7 + underset("orange")(2KOH) to underset("potassium chromate yellow")(2K_2CrO_4) + H_2O`
or `Cr_2O_7^(2-) + 2OH^(-) to underset(" yellow chromate ion)(2CrO_4^(-2)) + H_2O`
On acidifying the yellow solution, the colour again changes to orange red due to the reversible reaction.
`underset("yellow")(2K_2CrO_4) + H_2SO_4 to underset("orange")(K_2Cr_2O_7) + K_2SO_4 + H_2O`
or ` underset("yellow")(2CrO_4^(2-)) + 2H^(+) to underset("orange")(Cr_2O_7^(2-)) +H_2O`