Water of crystallisation is water that is chemically bonded into a crystal structure.

Here is the formula for hydrated copper(II) sulfate. Notice that water of crystallisation is separated from the main formula by a dot.

CuSO₄.5H₂O ← water of crystallisation

• hydrated means that the solid crystals contain water of crystallisation.
• dehydration is the removal of water of crystallisation.
• an anhydrous substance contains no water of crystallisation.
• the degree of hydration is the number of moles of water of crystallisation chemically bonded in 1 mole of the compound. The degree of hydration in the example above is 5.