An aq. pink colored solution of cobalt(II) chloride changes to deep blue on addition of excess of HCl. Why?
If an aqueous solution contains both cobalt(II) and chloride ions, the blue ion CoCl42- forms, in equilibrium with the pink Co2+ (aq) ion.
|CoCl42- (aq) <===========> Co2+ (aq) + 4Cl1-(aq)|
At relatively low concentrations of chloride, the equilibrium lies far to the right, and the solution is pink. If there is a large concentration of excess chloride, the equilibrium tends to the left, and the solution tends to be blue.