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  4. When concentrated HCl is added to an aqueous solution of CoCl2, its colour changes from reddish pink to deep blue. Which complex ion gives blue colour in this reaction ?

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Q. When concentrated $HCl$ is added to an aqueous solution of $CoCl_2$, its colour changes from reddish pink to deep blue. Which complex ion gives blue colour in this reaction ?

JEE MainJEE Main 2015The d-and f-Block Elements

Solution:

$\left[ COCl _{4}\right]^{-2}$ blue

$\left[ CO \left( H _{2} O \right)_{6}\right]^{+2}$ pink $]$