Copper sulfate pentahydrate (CuSO₄·5H₂O), the blue crystalline form you typically encounter, changes mass when heated due to the loss of water molecules. Here's why:

* Water of Crystallization: Copper sulfate pentahydrate contains five water molecules bound to each copper sulfate molecule. These water molecules are not chemically bonded but rather held within the crystal structure through weak forces.

* Dehydration: When you heat copper sulfate pentahydrate, you provide energy to break these weak bonds. The water molecules escape as water vapor, leaving behind anhydrous copper sulfate (CuSO₄).

* Color Change: Anhydrous copper sulfate is a white powder, contrasting with the blue color of the hydrated form. This color change is a visual indicator of the dehydration process.

* Mass Loss: Since water molecules are lost during heating, the mass of the sample decreases. The amount of mass lost corresponds to the mass of the water that was evaporated.

Here's a simple equation representing the process:

CuSO₄·5H₂O (blue, hydrated) → CuSO₄ (white, anhydrous) + 5H₂O (water vapor)

Key Points:

* The change in mass is directly related to the amount of water lost.

* The process is reversible; you can rehydrate the anhydrous copper sulfate by adding water, and it will turn blue again.

* This property of copper sulfate is utilized in various applications, including as a desiccant (drying agent) and in chemical experiments to illustrate the concept of hydration and dehydration.