When Co(II) cholride is dissolved in concentrated HCl, a blue solution is obtained. Upon dilution with water, the colour changes to pink. Why?

dear madhumita,

If an aqueous solution contains both cobalt(II) and chloride ions, the blue ion CoCl₄^2- forms, in equilibrium with the pink Co²⁺ (aq) ion.

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.

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