An orange-red aqueous solution of a transition metal salt 'X' changes its colour to yellow when the pH is raised above 7, forming species 'Y'. Upon lowering the pH below 4, the yellow solution turns orange-red again, regenerating 'X'. The oxidation states of the transition metal in species 'X' and 'Y', respectively, are :

Step-by-step solution

The colour changes and pH dependence uniquely identify the chromate-dichromate equilibrium. The orange-red species 'X' present at low pH is the dichromate ion ( Cr ₂ O ₇²⁻ ). The yellow species 'Y' present at high pH is the chromate ion ( CrO ₄²⁻ ). The equilibrium is given by: Cr ₂ O ₇²⁻ + 2 OH ^- 2 CrO ₄²⁻ + H ₂ O In Cr ₂ O ₇²⁻ , let the oxidation state of Cr be x . 2x + 7(-2) = -2 2x = 12 x = +6 In CrO ₄²⁻ , let the oxidation state of Cr be y . y + 4(-2) = -2 y = +6 Thus, the oxidation state of chromium remains