The aqueous solution of transition metal salt changes colour from pink to blue, when concentrated hydrochloric acid is added to it. The change in colour is due to

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formation of a coordination complex of the metal ion with hydrochloric acid.

protonation of the metal ion.

change in the coordination number of the metal ion from 6 to 4 and formation of new species in solution.

evolution of hydrogen that changes the oxidation state of the metal ion.

answer is B.

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Detailed Solution

In transition metals, ${[M (H_{2} O)]}^{6 +}$ (M=transition metals) has pink color and has a coordination number, 6. When this complex is treated with conc. HCl, then its coordination number changes from 6 to 4 and form a new complex, ${[{MCl}_{4}]}^{-}$ . The color of the complex becomes blue.