Question

When orange solution containing \[\ce{Cr2O^{2-}7}\] ion is treated with an alkali, a yellow solution is formed and when \[\ce{H^+}\] ions are added to yellow solution, an orange solution is obtained. Explain why does this happen?

Answer 1

This is due to the interconversion of dichromate (orange) to chromate ion (yellow). 

\[\ce{\underset{\underset{(Orange)}{Dichromate}}{Cr2O^{2-}7} ⇌[OH^-][H^+] \underset{\underset{(Yellow)}{Chromate}}{CrO^{2-}4}}\]