An aqueous solution of CoCl ₂ on addition of excess of concentrated HCl turns blue due to formation of

Step-by-step solution

CoCl ₂ is a weak Lewis acid, reacting with chloride ion to produce salt containing the tetrahedral [ CoCl ₄ ]²⁻ ion. 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.