What is water of crystallisation?

• A. Water included in the structure of certain salts as they crystallise
• B. Water that dissolves salts
• C. Water used to wash crystals
• D. Water obtained after drying a salt

Answer: (A)

Water of crystallisation is water molecules that are chemically bound inside the crystal structure of certain salts. When a salt forms crystals from solution, some of the water becomes an integral part of the solid, not just free in solution. A classic example is copper sulfate, which can be written as CuSO4·5H2O—five water molecules are attached to each formula unit in the crystal. These water molecules are held in place in the lattice and are released as water vapour when the salt is heated, leaving the anhydrous salt behind. This concept is different from water that simply dissolves the salt in a solvent, or water used to wash crystals, or water that comes off when drying a salt.