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  4. An aqueous solution of CoCl2 on addition of excess of cone. HCl turns blue due to the formation of

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Q. An aqueous solution of $CoCl_{2}$ on addition of excess of cone. $ HCl $ turns blue due to the formation of

Delhi UMET/DPMTDelhi UMET/DPMT 2008

Solution:

$CoCl _{2}$, a weak Lewis acid, reacts with chloride ion to produce salt containing the tetrahedral, $\left[ CoCl _{4}\right]^{2-}$ ion. It is blue when anhydrous and a deep magenta colour when hydrared, for this reason it is widely used as an indicator for water.