When an orange solution containing $Cr_{2}O_{7}^{2-}$ 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 orange-colored $Cr_{2}O_{7}^{2-}$ ion and the yellow-colored $CrO_{4}^{2-}$ ion are interconvertible depending on the $pH$ of the medium.
In an alkaline medium,the equilibrium shifts towards the formation of chromate ions:
$Cr_{2}O_{7}^{2-} + 2OH^{-} \rightleftharpoons 2CrO_{4}^{2-} + H_{2}O$
(Orange) $\quad$ (Yellow)
In an acidic medium,the equilibrium shifts towards the formation of dichromate ions:
$2CrO_{4}^{2-} + 2H^{+} \rightleftharpoons Cr_{2}O_{7}^{2-} + H_{2}O$
(Yellow) $\quad$ (Orange)