Why is it that an orange solution of K$_2$Cr$_2$O$_7$ turns yellow on adding NaOH to it?

CBSE Class 12 · 2000 · All India Compartment · d and f Block ElementsVery Short Answer

Why is it that an orange solution of K

_{2}

_{2}

_{7}

turns yellow on adding NaOH to it?

Answer: In alkali, dichromate converts to yellow chromate: Cr₂O₇²⁻ + 2OH⁻ → 2CrO₄²⁻ + H₂O.

Dichromate and chromate exist in a pH-dependent equilibrium.
Adding NaOH shifts it to the chromate side:

C r_{2} O_{7}^{2 -} (orange) + 2O H^{-} \to 2Cr O_{4}^{2 -} (yellow) + H_{2} O

Hence the solution turns yellow (acid restores the orange dichromate).

CBSE Class 12 Board · 2000 · All India Compartment · Subjective