The Mystery of Hydration – Why Salts Change Colour 

Ever noticed how some crystals look bright blue when fresh from the bottle, but turn white after heating? That colour change is the mystery of hydration — and it makes chemistry come alive in the classroom.

---

💧 What is a Hydrated Salt?

Many salts form crystals that trap water molecules inside their structure. These are called hydrated salts. The water is chemically bound, not just sitting on the surface.

Example: Copper(II) sulfate pentahydrate (CuSO₄·5H₂O) is bright blue.

---

🔥 Heating the Crystals

When you gently heat hydrated copper sulfate, the water is driven off:

$$CuSO₄·5H₂O \;\; → \;\; CuSO₄ + 5H₂O$$

The blue crystals turn into a white powder. Add water back, and the colour returns — like chemical magic.

---

 Why Does Hydration Change Colour?

The key lies in how water molecules interact with metal ions inside the crystal.

1. Hydrated salts (like copper(II) sulfate pentahydrate, CuSO₄·5H₂O) contain water molecules coordinated (bonded) directly to the central metal ion.

   • In CuSO₄·5H₂O, four water molecules form a complex with the Cu²⁺ ion, creating [Cu(H₂O)₄]²⁺.

2. These water ligands change the electronic environment of the Cu²⁺ ion.

   • The water molecules split the copper’s d-orbitals into slightly different energy levels.

   • When visible light hits the crystal, electrons in copper can absorb specific wavelengths to jump between these split orbitals.

   • The remaining wavelengths of light are transmitted or reflected, giving hydrated copper sulfate its intense blue colour.

3. When you heat the crystals, the water of crystallisation is driven off.

   • Without the water ligands, the copper ions sit in a different environment (often surrounded only by sulfate ions).

   • The orbital splitting changes, so the ions absorb different wavelengths — and the compound appears almost colourless or pale white.

4. Add water back, and the hydrated complex reforms, restoring the blue colour.

---

🔬 In Other Salts

• Cobalt(II) chloride is another classroom favourite. Hydrated CoCl₂·6H₂O is pink, but when dehydrated it turns blue. Again, the colour depends on whether water ligands surround the Co²⁺ ions.

• This principle — called ligand field theory — is part of transition metal chemistry, explaining why so many metal complexes are vibrantly coloured.

---

🧪 Classroom Connections

• Thermal decomposition vs dehydration – not all heating breaks bonds in the same way.

• Reversible reactions – add water and the colour comes back.

• Real-world links – cobalt chloride paper changes colour when it absorbs water, and hydrated salts are used in desiccants and even in hand-warmers.

---

🎓 Student Takeaway

The “mystery” isn’t really magic — it’s chemistry. But watching a salt change colour before your eyes shows how structure, bonding, and water molecules combine to make something memorable.