In an acidified solution of ${\mathrm{K}}_2{\mathrm{Cr}}_2{\mathrm{O}}_7,{\mathrm{H}}_2{\mathrm{O}}_2$ is added. Then,

Hydrogen peroxide acts as an oxidising as well as a reducing agent in both acidic and alkaline media. 

When H₂O₂ is added to an acidified potassium dichromate solution, the blue coloured compound CrO₅ or CrO(O₂)₂ is formed. The chemical reaction is as follows

${\mathrm{Cr}}_2{\mathrm{O}}_7^{2-}+2{\mathrm{H}}^{+}+4{\mathrm{H}}_2{\mathrm{O}}_2\to 2\mathrm{CrO}{\left({\mathrm{O}}_2\right)}_2+5{\mathrm{H}}_2\mathrm{O}$