Hydrated – IGCSE Chemistry Definition

A hydrated substance is a crystalline compound that contains water of crystallisation within its crystal structure. The water molecules are chemically bonded or coordinated into the lattice and are present in a fixed ratio. The formula of a hydrated salt is written with a dot separating the salt formula from the number of water molecules, for example CuSO₄·5H₂O (hydrated copper(II) sulfate, which contains 5 moles of water per mole of CuSO₄).

Water of crystallisation

Water of crystallisation is the specific number of water molecules that are incorporated into the crystal lattice of a salt when it crystallises from solution. These water molecules affect the appearance and properties of the crystal. For example, hydrated copper(II) sulfate (CuSO₄·5H₂O) is blue, while anhydrous copper(II) sulfate (CuSO₄, with no water) is white.

Removing water of crystallisation

Heating a hydrated salt drives off the water of crystallisation, converting it to the anhydrous form. The mass decreases because water is lost. This process is used in calculations to determine the value of x in the formula of a hydrated salt.

Exam context

Paper 4 questions frequently involve heating a known mass of hydrated salt, measuring the mass loss, and calculating the number of moles of water per mole of salt. Candidates must be able to work with the dot formula and understand what it represents.

Worked exam question

A student heats 6.25 g of hydrated copper(II) sulfate, CuSO₄·xH₂O, until all the water is removed. The mass of the anhydrous salt remaining is 4.00 g. Calculate x. (Mr: CuSO₄ = 160, H₂O = 18) (3 marks)

Mass of water lost = 6.25 - 4.00 = 2.25 g [1]. Moles of CuSO₄ = 4.00/160 = 0.025 mol. Moles of H₂O = 2.25/18 = 0.125 mol [1]. Ratio = 0.125/0.025 = 5, so x = 5. Formula is CuSO₄·5H₂O [1].

Common mistakes

Candidates sometimes assume the mass lost during heating is the mass of the salt rather than the mass of water. The mass lost is always the water of crystallisation. Another error is forgetting to use the anhydrous Mr (without water) when calculating moles of the salt. Using the hydrated Mr leads to a circular calculation.