Final answer:
The complex ion responsible for the deep blue color when concentrated HCl is added to CoCl₂ solution is [CoCl₄]²⁺. This reaction is reversible and an example of how transition metal ions form different colored complexes.
Step-by-step explanation:
When concentrated HCl is added to an aqueous solution of CoCl₂, a reversible reaction takes place where the reddish pink color of the solution, indicative of the [Co(H₂O)₆]²⁺ complex, changes to deep blue. This deep blue color indicates the formation of the [CoCl₄]²⁺ complex ion. This reaction demonstrates the concept of dynamic equilibrium in a reversible reaction and the ability of transition metal ions to form various colored complexes. The complex ion responsible for the blue color is [CoCl₄]²⁺, as more chloride ions from HCl coordinate with the cobalt ion, displacing water molecules and forming the blue complex.
This color change is connected to the hydration state of cobalt chloride; the anhydrous form is indeed blue due to the [CoCl₄]²⁺ complex, and it can serve as a humidity indicator. In the hydrated state, when the cobalt ion is surrounded by water molecules, the complex is pink. Additionally, the addition of other ligands such as ammonia can lead to similar dramatic color changes in transition metal complexes, as exampled by the change in color of hydrated copper ion solutions upon addition of ammonia.