Which of the following statements is incorrect?

The correct answer is Option (3) → during oxidizing action in an acidic solution, \({Cr_2O_7}^{2-}\), gets converted to \(Cr^{2+}\)

In acidic solution the dichromate ion is a strong oxidising agent. Each chromium is reduced from +6 to +3, taking up three electrons, so the green \( \ce{Cr^3+} \) ion is formed, not \( \ce{Cr^2+} \):

    \( \displaystyle \ce{Cr2O7^2- + 14H^+ + 6e^- -> 2Cr^3+ + 7H2O} \qquad (E^\circ = 1.33\ \text{V}) \)

The oxidation state of Cr in \( \ce{Cr2O7^2-} \): 2x + 7(−2) = −2, so x = +6. \( \ce{Cr^2+} \) is itself a strong reducing agent and is not the product of an oxidising action.

• Option (1) chromate ion has tetrahedral geometry — A correct statement. In \( \ce{CrO4^2-} \) four oxygens surround the chromium tetrahedrally; the dichromate ion is two such tetrahedra sharing one corner (Cr–O–Cr angle 126°).

• Option (2) sodium and potassium dichromates are strong oxidising agents — A correct statement. Both oxidise, for example, iodide to iodine and \( \ce{Fe^2+} \) to \( \ce{Fe^3+} \); the more soluble sodium salt is widely used in organic chemistry.

• Option (3) during oxidizing action in an acidic solution, \( {Cr_2O_7}^{2-} \), gets converted to \( Cr^{2+} \) — Not a correct statement. Dichromate is reduced to Cr³⁺, not Cr²⁺.

• Option (4) potassium dichromate is used as a primary standard in volumetric analysis. — A correct statement. It is obtained very pure, is not hygroscopic and keeps unchanged, so its standard solution can be made by direct weighing.

NCERT (2026-27 edition):
Oxides and Oxoanions of Metals – …

Oxides and Oxoanions of Metals – “… The structures of chromate ion, CrO₄^2– and the dichromate ion, Cr₂O₇^2– are shown below. The chromate ion is tetrahedral whereas the dichromate ion consists of two tetrahedra sharing one corner with Cr–O–Cr bond angle of 126°.”