Chromium(III) hydroxide is warmed with Na2O2 in alkaline medium: 2Cr(OH)3 + 3Na2O2 -> 2Na2CrO4 + 2NaOH + 2H2O. The chromium-containing product has chromium in oxidation state and colour:

JEE Main levelSingle correct optionDifficulty 2 of 3sb129

Chromium(III) hydroxide is warmed with $Na X_{2} O X_{2}$ in alkaline medium: $2 Cr (OH) X_{3} + 3 Na X_{2} O X_{2} 2 Na X_{2} CrO X_{4} + 2 NaOH + 2 H X_{2} O$ . The chromium-containing product has chromium in oxidation state and colour:

Peroxide oxidises

Cr (III)

to chromate, so Cr changes from

+ 3

+ 6

(a net 3 electrons per Cr, matching 3 peroxides for 2 Cr). In basic solution the species is

CrO X_{4} X^{2 -}

, which is yellow; orange

Cr X_{2} O X_{7} X^{2 -}

forms only in acid.