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

  • A
    evolution of hydrogen that changes the oxidation state of the metal ion
  • B
    change in the coordination number of the metal ion from $6$ to $4$ and formation of new species in solution
  • C
    formation of a coordination complex of the metal ion with hydrochloric acid
  • D
    protonation of the metal ion

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