The colour change of K 2 Cr 2 O 7 from orange to yellow on adding NaOH is due to:

Cr 2 O 7 2- (orange, acidic) converting to CrO 4 2- (yellow, basic)

A reduction of chromium from the +6 to the +3 oxidation state

An oxidation of chromium to an even higher oxidation state than +6

A simple temperature change caused by mixing the two solutions

The colour change of K 2 Cr 2 O 7 from orange to yellow on…

Answer: Cr₂O₇^2- (orange, acidic) converting to CrO₄^2- (yellow, basic).

A Cr₂O₇^2- (orange, acidic) converting to CrO₄^2- (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. Cr₂O₇^2- (orange, acidic) converting to CrO₄^2- (yellow, basic)

Explanation: Cr₂O₇^2- + 2OH- → 2CrO₄^2- + H₂O. 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.

Read the full d- and f-Block Elements notes →

The colour change of K2Cr2O7 from orange to yellow on adding NaOH is due to:

Concept context

More d- and f-Block Elements questions

The colour change of K₂Cr₂O₇ from orange to yellow on adding NaOH is due to: