What is the colour of copper sulphate? 

Blue 

The correct answer is option 4. Blue.

Copper sulfate is a chemical compound with the formula CuSO₄. It exists in several hydrated forms, and the most common one is copper sulfate pentahydrate (CuSO₄·5H₂O), which is a bright blue crystalline solid. The blue color of copper sulfate is a characteristic feature, and it arises from the presence of copper ions (Cu²⁺) in the compound.

The blue color of copper sulfate is due to the absorption of certain wavelengths of light by the copper ions in the crystal lattice. The complex structure of copper sulfate molecules, along with water molecules in the hydrated forms, contributes to the observed color.

When copper sulfate dissolves in water, it forms a blue solution. The intensity of the blue color may vary with the concentration of the solution and the specific form of copper sulfate present.

In various applications, copper sulfate is used for its distinctive blue color, such as in laboratory experiments, agriculture as a fungicide, and in decorative applications. It's important to note that while the compound itself is blue, the hydrated forms may appear more or less intense in color depending on the water content.