When ${\mathrm{H}}_2{\mathrm{O}}_2$ is shaken with an acidified solution of ${\mathrm{K}}_2{\mathrm{Cr}}_2{\mathrm{O}}_7$ in presence of ether, the ethereal layer turns blue due to the formation of

Acidified ${\mathrm{K}}_2{\mathrm{Cr}}_2{\mathrm{O}}_7$ solution reacts with hydrogen peroxide to give a deep blue solution due to the formation of peroxo compound ${\mathrm{CrO}}_5\text{. }$

${\mathrm{K}}_2{\mathrm{Cr}}_{2 }{\mathrm{O}}_7+4{\mathrm{H}}_2{\mathrm{O}}_2+{\mathrm{H}}_2{\mathrm{SO}}_4⟶2{\mathrm{CrO}}_5+{\mathrm{K}}_2{\mathrm{SO}}_4+5{\mathrm{H}}_2\mathrm{O}$