3.10 - Water of Crystallisation

Hydrated and anhydrous substances

• Some solids have water built into their crystals. In these solids, water molecules are fitted into the lattice alongside the ions.
• Each water molecule is attracted to the surrounding ions and held in place.
• The water isn't damp on the surface - it's chemically combined as a fixed part of the solid.

Hydrated vs anhydrous

• Hydrated - A substance whose crystals contain water of crystallisation.
• Anhydrous - A substance containing no water at all.
• Hydrated and anhydrous copper(II) sulfate are the same salt, with and without that water: the hydrated form is blue and the anhydrous form is white.

Water counts towards the mass

Because the water is part of the solid, it's included in the mass of the crystals when weighed on a balance.