Hydrated & Anhydrous Salts
Section: 7. Acids, Bases & Salts | Syllabus: Cambridge IGCSE Chemistry (0620)
Hydrated and Anhydrous Substances
- A hydrated substance is chemically combined with water (water of crystallisation)
- An anhydrous substance contains no water
Common hydrated salts: CuSO₄·5H₂O (blue), CoCl₂·6H₂O (pink), MgSO₄·7H₂O (Epsom salt), Na₂CO₃·10H₂O (washing soda).
Water of Crystallisation
Water of crystallisation is the water molecules chemically combined within a hydrated crystal, in a fixed ratio to the salt. The dot (·) in the formula shows this: e.g. CuSO₄·5H₂O and CoCl₂·6H₂O are the two examples named in the syllabus.
| Salt | Hydrated colour | Anhydrous colour |
|---|---|---|
| CuSO₄·5H₂O / CuSO₄ | blue | white |
| CoCl₂·6H₂O / CoCl₂ | pink | blue |
Heating a hydrated salt drives off its water of crystallisation, leaving the anhydrous salt (see the Reversible Reactions topic for the full colour-change demonstration with these two salts).
Calculating Water of Crystallisation
The value of x in a formula CuSO₄·xH₂O can be found from mass measurements before and after heating.
Worked Example
- Question: 12.5 g of hydrated copper(II) sulfate, CuSO₄·xH₂O, is heated until all the water is driven off, leaving 8.0 g of anhydrous CuSO₄. Calculate x. (Ar: Cu = 64, S = 32, O = 16, H = 1) [4]
- Answer: mass of water lost = 12.5 − 8.0 = 4.5 g
- moles CuSO₄ = 8.0 ÷ 160 = 0.05 mol; moles H₂O = 4.5 ÷ 18 = 0.25 mol
- ratio H₂O : CuSO₄ = 0.25 : 0.05 = 5 : 1, so x = 5 and the formula is CuSO₄·5H₂O
Mass lost on heating tells you the moles of water; comparing this to moles of anhydrous salt gives the formula ratio
Common Mistakes
- ❌ Confusing water of crystallisation with ordinary surface moisture
- ✓ water of crystallisation is chemically part of the crystal structure, in a fixed ratio - it only comes off on heating, not just by air-drying
- ❌ Using the mass of the hydrated salt (not the water lost) to find moles of water
- ✓ first subtract the anhydrous mass from the hydrated mass to isolate the mass of water, then convert that to moles
Interactive revision notes, videos and practice questions load below.