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(C) When $NaOH(aq)$ is added to $Cr^{3+}(aq)$,a green-blue precipitate of chromium $(III)$ hydroxide is formed: $Cr^{3+}(aq) + 3OH^-(aq) \rightarrow [

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$[Cr(OH)_4]^-$

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(C) When $NaOH(aq)$ is added to $Cr^{3+}(aq)$,a green-blue precipitate of chromium $(III)$ hydroxide is formed: $Cr^{3+}(aq) + 3OH^-(aq) \rightarrow [Cr(OH)_3(H_2O)_3](s)$ (green-blue precipitate). On adding excess $NaOH$,the precipitate dissolves to form a soluble complex ion: $[Cr(OH)_3(H_2O)_3](s) + OH^-(aq) \rightarrow [Cr(OH)_4(H_2O)_2]^-(aq)$ (green solution). This complex is often represented as $[Cr(OH)_4]^-$. Therefore,the green colour is due to the formation of the $[Cr(OH)_4]^-$ ion.

When aqueous sodium hydroxide is added to an aqueous solution of chromium $(III)$ ions,a green-blue precipitate is first formed which re-dissolves to give a green solution. This green colour is due to

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