In chemistry, water of crystallisation refers to the water molecules that are tightly bound within a crystal lattice. Think of them as tiny water “guests” that sit inside the solid structure, holding the crystal together like beads on a string. These water molecules are not part of the main chemical formula but are essential to the crystal’s shape and stability. When the crystal is heated, the water can evaporate, leaving behind a dry salt.
Example 1: 🧪 Copper(II) sulphate pentahydrate – written as \$\mathrm{CuSO4 \cdot 5H2O}\$. Here, five water molecules are attached to each copper sulphate unit.
Example 2: 🧪 Cobalt(II) chloride hexahydrate – written as \$\mathrm{CoCl2 \cdot 6H2O}\$. Six water molecules surround each cobalt chloride unit.
| Compound | Formula | Water Molecules |
|---|---|---|
| Copper(II) sulphate | \$\mathrm{CuSO4 \cdot 5H2O}\$ | 5 |
| Cobalt(II) chloride | \$\mathrm{CoCl2 \cdot 6H2O}\$ | 6 |
When you see a hydrated salt in a question, double‑check whether the water of crystallisation is required in the final answer. If the question asks for the dry salt, remove the “\$\cdot nH_2O\$” part. If it asks for the hydrated salt, include it. A quick mental check: “Is the salt solid or dissolved?” Solid → include water; Dissolved → usually ignore the water of crystallisation.