An aqueous solution of potassium dichromate is orange in colour. On...

In aqueous solution, dichromate ions and chromate ions exist in equilibrium depending on the pH of the solution.

$Cr_2O_7^{2-} + 2OH^- \rightleftharpoons 2CrO_4^{2-} + H_2O$

The dichromate ion ($Cr_2O_7^{2-}$) is orange in colour, while the chromate ion ($CrO_4^{2-}$) is yellow. On adding an alkali, the concentration of $OH^-$ increases, shifting the equilibrium to the right and forming yellow $CrO_4^{2-}$ ions.

Answer: $CrO_4^{2-}$