2.3.4—Terms: anhydrous, hydrated and water

An anhydrous salt contains no water of crystallisation in its crystal structure. A hydrated salt contains a fixed number of water molecules associated with each formula unit; the water is part of the crystal composition and is written after a dot in the hydrate formula.

Water of crystallisation is not simply liquid solvent or surface moisture. It is held in a definite stoichiometric ratio, so a hydrate can be represented as salt · nH₂O, where n is the number of water molecules per formula unit in that crystal.

Heating a hydrated salt can remove its water of crystallisation and form the anhydrous salt. The salt formula and its recorded mass therefore change together; use the stated hydrate ratio rather than treating the water content as arbitrary.

Do not interpret hydrated as dissolved, wet or chemically bonded into a new molecular compound, and do not assume every salt has water of crystallisation. The quantitative mass-loss calculation belongs to the neighbouring formula-calculation objective.