Explain why complexes of transition elements are frequently coloured.
After water is added to white anhydrous $\text{CuSO}_4$, the solid dissolves and a blue solution is obtained. When concentrated $\text{NH}_4\text{Cl(aq)}$ is then introduced, the solution turns yellow-green. Further concentration gives green crystals of an ammonium salt with the empirical formula $\text{CuN}_2\text{H}_8\text{Cl}_4$. Explain these observations, showing your reasoning.
Copper may be obtained from low-grade ores by ‘leaching’ the ore with dilute $\text{H}_2\text{SO}_4$, which changes the copper compounds in the ore into $\text{CuSO}_4\text{(aq)}$. The copper concentration in the leach solution can then be estimated by adding an excess of aqueous potassium iodide, and titrating the iodine produced with standard $\text{Na}_2\text{S}_2\text{O}_3\text{(aq)}$. Reactions: $2\text{Cu}^{2+} + 4\text{I}^- \rightarrow 2\text{CuI} + \text{I}_2$ $\text{I}_2 + 2\text{S}_2\text{O}_3^{2-} \rightarrow 2\text{I}^- + \text{S}_4\text{O}_6^{2-}$ When excess of $\text{KI(aq)}$ was added to a $50.0\,\text{cm}^3$ portion of leach solution, and the mixture was titrated, $19.5\,\text{cm}^3$ of $0.0200\,\text{mol dm}^{-3}\,\text{Na}_2\text{S}_2\text{O}_3\text{(aq)}$ were needed to remove the iodine colour. Calculate the $[\text{Cu}^{2+}\text{(aq)}]$, and hence the percentage by mass of copper, in the leach solution.