Crystal Water Hunt

Why it matters

Heat bright blue crystals until they crumble into a ghostly white powder. The water didn't evaporate from the surface; it was trapped inside the crystal structure.

The idea in plain words

Balance the equation — find the coefficient on H₂O: CuSO₄·5H₂O → CuSO₄ + 5H₂O.

Predict first

What happens to the water molecules when you heat hydrated copper sulphate?

The heat breaks the weak bonds holding water in the lattice. The blue hydrated salt becomes white anhydrous salt and water vapour.

• They turn into steam and escape, leaving white anhydrous salt — correct
• They decompose into hydrogen and oxygen gas
• They remain trapped inside the white powder
• The copper sulphate decomposes into copper metal and sulphur dioxide

What you do

Heat the blue crystals. Observe the colour change to white. Adding a few drops of water back turns the white powder blue again — that reversible colour change is the chemist's test for water.

Check yourself

Why does the colour change from blue to white upon heating?

The blue colour is due to hydrated copper ions. Removing the water (dehydration) leaves white anhydrous copper sulphate.

If you add water to the white powder and it turns blue again, what does this prove?

The reversible colour change is a standard test for the presence of water. The white anhydrous salt re-absorbs water to become blue hydrated salt.

Which equation correctly represents the heating of hydrated copper sulphate?

Heating causes thermal decomposition where the hydrate breaks down into anhydrous salt and water vapour.

• The copper ions are destroyed
• The water molecules responsible for the blue colour are removed — correct
• The salt turns into a different chemical element
• The white powder was actually just dirty blue crystals
• Water is present in the original sample as water of crystallisation — correct
• The water reacted with the air to form blue paint
• CuSO₄ + 5H₂O → CuSO₄·5H₂O
• CuSO₄·5H₂O → CuSO₄ + 5H₂O — correct
• CuSO₄·5H₂O → Cu + SO₄ + 5H₂O