An aqueous solution of ${\mathrm{CoCl}}_2$ on addition of excess of concentrated $HCI$ turns blue due to formation of

${\mathrm{CoCl}}_2$ is a weak Lewis acid, reacting with chloride ion to produce salt containing the tetrahedral ${\left[{\mathrm{CoCl}}_4\right]}^{2-}$ ion. ${\mathrm{CoCl}}_2$ is blue when anhydrous, and a deep magenta colour when hydrated, for this reason it is widely used as an indicator for water. ,