Water of Crystallisation

What is Water of Crystallisation?

The water molecules present in hydrated crystals. They are chemically combined in the crystal in a fixed number per formula unit, which is why the formula of a hydrated salt states that number after a dot.

This definition is part of the Acids, Bases and Salts chapter in Cambridge O Level Chemistry.

Concepts related to Water of Crystallisation

• hydrated crystal
• fixed ratio
• formula dot

Common mistakes with Water of Crystallisation

• “Water of crystallisation is water clinging to the outside of the crystals.” Repair Surface water is damp crystals, and it comes off by drying between filter papers. Water of crystallisation is chemically combined inside the crystal, in a fixed number of molecules per formula unit, and removing it changes the substance — blue crystals become a white powder.