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  4. Assertion: Aq. solution of CoCl2 is pink in colour. It turns blue in presence of conc. HCl. Reason: It is due to the formation of [CoCl4]2-.

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Q. Assertion : Aq. solution of $CoCl_2$ is pink in colour. It turns blue in presence of conc. $HCl$.
Reason : It is due to the formation of $[CoCl_4]^{2-}$.

AIIMSAIIMS 2013Equilibrium

Solution:

$\left[ Co \left( H _{2} O \right)_{6}\right]^{2+}+4 Cl ^{-} \rightleftharpoons\left[ CoCl _{4}\right]_{\text {Blue }}^{2-}+6 H _{2} O$
According to Le Chatelier's principle, on adding cone. $HCl$, the equilibrium shifts in forward direction giving blue colour. And when this blue colour is diluted, equilibrium shifts in backward direction leading to pink colour.