Concentrated sulphuric acid acts as a powerful dehydrating agent, removing the water of crystallisation from hydrated copper sulphate to produce anhydrous copper(II) sulfate.
When concentrated sulphuric acid is added to blue hydrated copper(II) sulphate crystals (CuSO₄·5H₂O), a chemical reaction occurs where the acid removes the water molecules that are chemically bound within the crystal structure. This process is known as dehydration.
Dehydration Process Explained
Sulphuric acid has a strong affinity for water molecules. In the case of hydrated copper sulphate, the acid 'pulls' the water out of the salt's crystal lattice. This action disrupts the structure responsible for the blue colour of the hydrated form.
Key Outcomes of the Reaction
- Removal of Water: The 5 molecules of water associated with each copper sulphate unit in the hydrated form are absorbed by the sulphuric acid.
- Formation of Anhydrous Salt: The result is the formation of copper(II) sulphate without the water of crystallisation. This is called anhydrous copper(II) sulphate.
- Colour Change: A striking visual change occurs. The vibrant blue colour of hydrated copper sulphate disappears, and the substance turns white or colourless, characteristic of anhydrous copper(II) sulphate.
- Exothermic Reaction: The process of hydration of concentrated sulphuric acid is exothermic, meaning it releases heat. While the primary action here is dehydration of the salt, interaction with any freed water molecules contributes to potential heat generation.
Summary Table
Here's a simple breakdown of the change:
| Property | Hydrated Copper Sulphate (Reactant) | Anhydrous Copper Sulphate (Product) |
|---|---|---|
| Formula | CuSO₄·5H₂O | CuSO₄ |
| Colour | Blue | Colourless/White |
| Water Content | Contains water of crystallisation | Water removed |
The chemical equation representing the dehydration is often simplified to show the removal of water:
CuSO₄·5H₂O(s) + Concentrated H₂SO₄ → CuSO₄(s) + 5H₂O (absorbed by H₂SO₄)
Reversibility
The reaction is reversible. If water is added back to anhydrous copper(II) sulphate, it will absorb the water, turn blue again, and reform hydrated copper sulphate. This property is sometimes used as a test for the presence of water.
In summary, concentrated sulphuric acid acts specifically as a dehydrating agent on hydrated copper sulphate, stripping it of its water content and causing a distinct colour change from blue to white.
