Hydrogen peroxide oxidizes chromium(III) in alkaline solution.

The dilute solution of chromium(III) sulfate reacts with the excess of sodium hydroxide solution. The resulting solution is green and becomes brownish when 3% hydrogen peroxide is added. Brown colour is due to the presence of chromium(IV or V) peroxide compounds. Slowly these compounds decompose and the colour turns yellow. The right glass contains diluted chromate solution for comparison.