When dilute hydrochloric acid is added to sodium chromate solution the colour changes to:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → orange
Sodium chromate solution is yellow (\( \ce{CrO4^{2-}} \)). On adding acid, chromate changes into dichromate, which is orange:
\( \ce{2CrO4^{2-} + 2H^+ -> Cr2O7^{2-} + H2O} \)
The change is not a redox reaction: Cr stays in the +6 state in both ions. Chromate and dichromate are interconverted by changing the pH (alkali turns dichromate back to yellow chromate).
NCERT (2026-27 edition):
Oxides and Oxoanions of Metals – “… The yellow solution of sodium chromate is filtered and acidified with sulphuric acid to give a solution from which orange sodium dichromate, Na2Cr2O7.2H2O can be crystallised.”
Oxides and Oxoanions of Metals – “… The chromates and dichromates are interconvertible in aqueous solution depending upon pH of the solution. The oxidation state of chromium in chromate and dichromate is the same.”