What color change occurs for hydrated copper sulfate upon heating?

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

Answer: (A)

Hydrated copper sulfate is blue because the water molecules coordinated to the copper ions affect how light is absorbed. When you heat it, the water of crystallization is driven off, leaving the anhydrous form, which is white. So the observed change is blue to white as the compound becomes white without its water ligands. If you heat it much more, it can decompose further to copper oxide (which is black), but the standard drying observation is blue turning to white.