Copper sulphate crystals available in the market are blue coloured crystals. By careful heating, they turn to white colour. Which one of the following is responsible for the blue colour?

The most common form of copper sulphate is its pentahydrate, given by the chemical formula $\mathrm{CuSO}_4 \cdot 5 \mathrm{H}_2 \mathrm{O}$. It is a blue coloured chemical. When it is heated, the blue colour disappears and it turns white. Heating removes the water of crystallization which is responsible for blue colour of the crystal. $\begin{aligned} & \mathrm{CuSO}_4 \cdot 5 \mathrm{H}_2 \mathrm{O} \text { (Blue) }- \text { Heat } \longrightarrow \mathrm{CuSO}_4 \\ & \text {(White) }+5 \mathrm{H}_2 \mathrm{O}\end{aligned}$ After it cools, if a few drops of water are put into the test tube, the copper sulphate turns blue again as water of crystallization make the copper sulphate hydrated again.