Assertion (A): Chromates are converted into dichromates when the pH of their aqueous solution is raised.
Reason (R): The oxidation state of chromium is the same in the chromate ion and in the dichromate ion.
In the light of the above statements, choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (A) is false but (R) is true.
The yellow chromate ion and the orange dichromate ion change into each other in aqueous solution, and the direction of the change depends on the pH of the solution.
Assertion (A): Chromates are converted into dichromates when the pH of their aqueous solution is raised. — False. Chromate changes to dichromate when acid is added, that is, when the pH is lowered. Raising the pH (adding alkali) does the opposite and turns dichromate back into chromate:
\( \displaystyle \ce{2CrO4^2- + 2H+ -> Cr2O7^2- + H2O} \)
\( \displaystyle \ce{Cr2O7^2- + 2OH^- -> 2CrO4^2- + H2O} \)
Reason (R): The oxidation state of chromium is the same in the chromate ion and in the dichromate ion. — True. The change is not a redox reaction:
In CrO42−: x + 4(−2) = −2, so x = +6.
In Cr2O72−: 2x + 7(−2) = −2, so x = +6.
NCERT (2026-27 edition):
4.4.1 Oxides and Oxoanions of Metals – “Orange crystals of potassium dichromate crystallise out. 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.”
2 CrO42– + 2H+ → Cr2O72– + H2O
Cr2O72– + 2 OH- → 2 CrO42– + H2O