What is the colour of copper sulphate?

Blue

The correct answer is option 2. Blue.

Copper sulphate, also known as cupric sulfate or copper(II) sulphate, is a chemical compound with the formula \(CuSO_4\). It exists in several hydrated forms, the most common being the pentahydrate \(CuSO_4·5H_2O\), which is bright blue in color. This blue color is a distinctive characteristic of copper sulphate.

When copper sulphate pentahydrate crystals are dissolved in water, the resulting solution is also blue. This color arises due to the interaction between copper ions \((Cu^{2+})\) and water molecules. In the solution, the copper ions are surrounded by water molecules in a process called hydration. The absorption of light by the hydrated copper ions gives the solution its blue color.

Copper sulphate is widely used in various applications, including agriculture (as a fungicide and herbicide), chemistry (as a reagent), and in the manufacturing of other chemicals. Its blue color makes it easy to identify and distinguish from other compounds.

Overall, the blue color of copper sulphate is a result of its chemical structure and the interaction of copper ions with water molecules in solution.