MCQ Detailed View

Copper sulfate (CuSO₄) is an important inorganic compound widely used in laboratories, industries, and agriculture. In its crystalline form, it usually appears as copper sulfate pentahydrate (CuSO₄·5H₂O). These crystals have a characteristic bright blue color, which makes them easy to identify.

The blue color of copper sulfate crystals comes from the water of crystallization present in the compound. When copper sulfate combines with five molecules of water, it forms hydrated copper sulfate. The water molecules coordinate with the copper ions, causing an electronic transition within the copper ion’s d-orbitals. This transition absorbs specific wavelengths of light and reflects blue, which is why hydrated copper sulfate appears blue to the human eye.

If copper sulfate crystals are heated strongly, they lose their water of crystallization and become anhydrous copper sulfate (CuSO₄). In this form, the compound is a white powder instead of blue. This color change is often used in laboratories to demonstrate hydration and dehydration reactions. The white anhydrous form can also be used as a test for the presence of water, since it turns blue again when exposed to moisture.

Other colors listed in the options are incorrect. Copper sulfate is not brown or green in its crystalline state. Its most recognized form is the blue crystalline pentahydrate, which is the commercially and scientifically significant variety.

Copper sulfate crystals are also used in agriculture as a fungicide, in chemistry labs for experiments, and in education to teach about crystallization and hydration. Their distinctive blue color makes them one of the most well-known inorganic salts studied by students.

Thus, the crystalline form of copper sulfate is identified by its blue color, due to the presence of water of crystallization in CuSO₄·5H₂O.