What colour change occurs to hydrated copper sulfate upon heating?

• A. Blue to white
• B. Blue to green
• C. White to blue
• D. Blue to red

Answer: (A)

Hydrated copper sulfate is blue because water molecules are part of its crystal structure and coordinate with the copper(II) ions, giving that blue color. When you heat it, the water of crystallization is driven off, leaving anhydrous copper sulfate, which is white. So the substance changes from blue to white as it loses water. If you add water again, it rehydrates and the blue colour returns.