In acidic medium, ${\mathrm{Cr}}_2{\mathrm{O}}_7^{2-},$ changes to

In acidic condition, chromate ions are changed to dichromate ions.

$\underset{\text{ Yellow }}{2{{\mathrm{CrO}}_4}^{2-}+2{\mathrm{H}}^{+}}\to {\underset{\text{ Orange }}{{\mathrm{Cr}}_2{\mathrm{O}}_7}}^{2-}+{\mathrm{H}}_2\mathrm{O}$