When an orange solution containing $Cr_2O_7^{2-}$ ions is treated with an alkali,a yellow solution is formed,and when $H^{+}$ ions are added to the yellow solution,an orange solution is obtained. Explain why this happens.

(N/A) The interconversion of dichromate ($Cr_2O_7^{2-}$,orange) and chromate ($CrO_4^{2-}$,yellow) ions depends on the $pH$ of the solution.
The equilibrium reaction is:
$2CrO_4^{2-} (yellow) + 2H^{+} \rightleftharpoons Cr_2O_7^{2-} (orange) + H_2O$
When an alkali ($OH^{-}$ ions) is added,it reacts with $H^{+}$ ions,shifting the equilibrium to the left,which produces yellow $CrO_4^{2-}$ ions.
When $H^{+}$ ions are added,the concentration of $H^{+}$ increases,shifting the equilibrium to the right,which converts $CrO_4^{2-}$ back into orange $Cr_2O_7^{2-}$ ions.