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Aqueous $\mathrm{CuSO}_4$ changes its colour from sky blue to deep blue on addition of $\mathrm{NH}_3$ because
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$\left[\mathrm{Cu}\left(\mathrm{H}_2 \mathrm{O}\right)_4\right]^{2+}$ is labile complex and changes to $\left[\mathrm{Cu}\left(\mathrm{NH}_3\right)_4\right]^{2+}$ as $\mathrm{NH}_3$ is stronger ligand than $\mathrm{H}_2 \mathrm{O}$
As, $\mathrm{H}_2 \mathrm{O}$ is a weak field ligand and thus $\left[\mathrm{Cu}\left(\mathrm{H}_2 \mathrm{O}\right)_4\right]^{2+}$ absorbs red light in visible spectrum. But the colour appears blue because $\left[\mathrm{Cu}\left(\mathrm{H}_2 \mathrm{O}\right)_4\right]^{2+}$ is a labile complex and changes to $\left[\mathrm{Cu}\left(\mathrm{NH}_3\right)_4\right]^{2+}$, where $\mathrm{NH}_3$ is strong field ligand thus absorbs yellow light of visible spectrum and impart blue colour.
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