Blue copper(II) sulfate crystals are not dry in the way they look: every formula unit holds five water molecules in its structure. Heat them and the water leaves, the blue goes, and the mass drops by a predictable amount.
A hydrated substance is chemically combined with water; an anhydrous substance contains no water. Water of crystallisation is the water molecules present in the crystals of a hydrated salt, as in CuSO4·5H2O. Heating a hydrated salt drives off this water: blue hydrated copper(II) sulfate becomes white anhydrous copper(II) sulfate. Extended: the number of water molecules, x, can be found from the masses before and after heating.
Fig. 1: hydrated and anhydrous copper(II) sulfate
Section 01
Hydrated and anhydrous.
The dot in a formula such as CuSO4·5H2O shows water that is part of the crystal, in a fixed ratio.
Other hydrates: sodium carbonate, Na2CO3·10H2O (washing soda); magnesium sulfate, MgSO4·7H2O.
Test for water: water turns anhydrous copper(II) sulfate from white to blue, and blue cobalt(II) chloride to pink.
Examiner trap
Hydrated crystals look and feel dry: the water of crystallisation is locked inside the crystal structure, not wetting the surface. And removing it is a chemical change, shown by the colour change, not simply drying.
Stage 1 · Learn
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What does anhydrous mean?
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What colour is anhydrous copper(II) sulfate?
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Blue cobalt(II) chloride paper is used to test a liquid. It turns pink. This shows:
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Water of crystallisation is:
Section 02
Finding the formula.
Extended. Weigh the hydrated salt, heat it to constant mass, weigh again, and compare the moles of anhydrous salt with the moles of water lost.
Data: 2.50 g of CuSO4·xH2O is heated to constant mass, leaving 1.60 g of CuSO4 (Mr 160).
Water lost: 2.50 − 1.60 = 0.90 g, which is 0.90 ÷ 18 = 0.050 mol.
Anhydrous salt: 1.60 ÷ 160 = 0.010 mol.
Ratio: 0.050 ÷ 0.010 = 5, so x = 5 and the formula is CuSO4·5H2O.
Percentage of water: 90 ÷ 250 × 100 = 36% of hydrated copper(II) sulfate is water.
Stage 2 · Exam
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(Extended) 4.92 g of MgSO4·xH2O is heated to constant mass, leaving 2.40 g of MgSO4 (Mr 120). What is x?
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Why is a hydrated salt heated to constant mass when finding x?
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Water is added to white anhydrous copper(II) sulfate. What is observed?
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(Extended) What is the percentage by mass of water in Na2CO3·10H2O (Mr 286)?
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Hydrated and anhydrous salts, mastered.
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