An aqueous solution of potassium dichromate is orange in colour. On adding an alkali to this solution, the colour changes to yellow. This colour change is due to the formation of:

Step-by-step solution

In aqueous solution, dichromate ions and chromate ions exist in equilibrium depending on the pH of the solution. Cr₂O₇²⁻ + 2OH^- 2CrO₄²⁻ + H₂O The dichromate ion ( Cr₂O₇²⁻ ) is orange in colour, while the chromate ion ( CrO₄²⁻ ) is yellow. On adding an alkali, the concentration of OH^- increases, shifting the equilibrium to the right and forming yellow CrO₄²⁻ ions. Answer: CrO₄²⁻