Answer: Cr 2 O 7 2- (orange, acidic) converting to CrO 4 2- (yellow, basic).
- A Cr<sub>2</sub>O<sub>7</sub><sup>2-</sup> (orange, acidic) converting to CrO<sub>4</sub><sup>2-</sup> (yellow, basic)
- B A reduction of chromium from the +6 to the +3 oxidation state
- C An oxidation of chromium to an even higher oxidation state than +6
- D A simple temperature change caused by mixing the two solutions
Correct answer: A. Cr<sub>2</sub>O<sub>7</sub><sup>2-</sup> (orange, acidic) converting to CrO<sub>4</sub><sup>2-</sup> (yellow, basic)
Explanation: Cr<sub>2</sub>O<sub>7</sub><sup>2-</sup> + 2OH- → 2CrO<sub>4</sub><sup>2-</sup> + H<sub>2</sub>O. In acidic conditions dichromate (orange) is dominant; in basic conditions chromate (yellow) predominates.
Aqueous solutions of transition metal ions show characteristic colours caused by electrons absorbing visible light to jump between split d-orbitals; the exact colour depends on the metal, its oxidation state, and surrounding ligands.
Concept context
Transition metals (d-block) and lanthanides/actinides (f-block). Known for coloured compounds, variable oxidation states, complex formation, and catalytic properties. Frequently tested in JEE and NEET.