Why aqueous cobalt chloride solution when treated with conc. HCl...

Aqueous cobalt chloride solution contains the pink-colored octahedral complex $[\text{Co}(\text{H}_2\text{O})_6]^{2+}$.

When concentrated $\text{HCl}$ is added, the chloride ions act as ligands and replace the water molecules in the coordination sphere. The reaction is given by:

$[\text{Co}(\text{H}_2\text{O})_6]^{2+} + 4\text{Cl}^- \rightleftharpoons [\text{CoCl}_4]^{2-} + 6\text{H}_2\text{O}$

The complex $[\text{CoCl}_4]^{2-}$ has a coordination number of $4$. Since $\text{Cl}^-$ is a weak field ligand, the complex undergoes $sp^3$ hybridization, resulting in a tetrahedral geometry.

The formation of this tetrahedral $[\text{CoCl}_4]^{2-}$ complex is responsible for the deep blue color of the solution.

Answer: Due to formation of $[\text{CoCl}_4]^{2-}$ having tetrahedral geometry.