Question

Explain why is cobalt chloride pink in colour when dissolved in water but turns deep blue when treated with concentrated hydrochloric acid.

Answer 1

• The electronic configuration of ₂₇Co : [Ar] 3d¹4s² and Co²⁺[Ar] 3d¹. 
• When dissolved in water cobalt chloride, Co²⁺ forms pink complex, [Co(H₂O)₆]²⁺. 
• The complex has octahedral geometry. 
• Due to absorption of radiation in the visible region and d – d transition, it forms pink coloured solution. 
• When CoCl₂ solution is treated with concentrated HCl solution it turns deep blue. 
• This change is due to the formation of another complex, [CoCl₄]²⁺ which has a tetrahedral geometry. 
• Thus due to a change in geometry of the complex formed the colour of the solution changes from pink to deep blue.