Water of Crystallisation Explained: GCSE Chemistry

Water of crystallisation is water chemically bonded into the structure of certain ionic crystals. The familiar blue copper sulphate crystals (CuSO₄·5H₂O) contain five water molecules per formula unit. Heat the crystals and the water is driven off, leaving white anhydrous copper sulphate (CuSO₄) — a reversible change.

What is water of crystallisation?

Water of crystallisation is a fixed number of water molecules that are incorporated into the crystal lattice of an ionic compound when it crystallises from solution. These water molecules are not simply trapped — they are part of the crystal's regular, repeating structure, held in place by interactions with the ions.

A compound containing water of crystallisation is called a hydrated salt. The same compound without the water molecules is called anhydrous. The formula of a hydrated salt shows the number of water molecules per formula unit, written after a dot:

CuSO₄·5H₂O = copper(II) sulphate-5-water (blue crystals)

CuSO₄ = copper(II) sulphate (anhydrous — white powder)

What are common examples of hydrated salts?

The colour change in cobalt chloride (pink hydrated → blue anhydrous) is the basis of a common test for water: cobalt chloride paper turns from blue to pink in the presence of water.

How is water of crystallisation removed?

Water of crystallisation is removed by heating — a type of thermal decomposition. The water molecules gain enough energy to break free from the crystal lattice.

Example — dehydrating copper(II) sulphate:

CuSO₄·5H₂O → CuSO₄ + 5H₂O

The reaction is reversible: adding water to anhydrous CuSO₄ regenerates the blue hydrated crystals. This reversibility also makes anhydrous copper(II) sulphate a test for the presence of water — it turns blue when water is added.

Safety note: the hydration reaction releases energy as heat (it is exothermic). This is why plaster of Paris (CaSO₄, anhydrous) heats up when water is mixed in to set a plaster cast.

How do you calculate the formula of a hydrated salt?

Method: heat a known mass of hydrated salt until constant mass, then find the moles of salt and water lost and take the ratio.

Worked example: 8.01 g of hydrated copper(II) sulphate is heated until constant mass. The mass after heating is 5.12 g. Find the value of n in CuSO₄·nH₂O.

Step 1 — find the mass of water lost:

• Mass of water = 8.01 − 5.12 = 2.89 g

Step 2 — find the moles of each substance:

• Molar mass of CuSO₄ = 64 + 32 + (4 × 16) = 160 g/mol

• Moles of CuSO₄ = 5.12 ÷ 160 = 0.032 mol

• Molar mass of H₂O = (2 × 1) + 16 = 18 g/mol

• Moles of H₂O = 2.89 ÷ 18 = 0.1606 mol

Step 3 — find the ratio:

• Ratio = 0.1606 ÷ 0.032 = 5.02 ≈ 5

Answer: n = 5, so the formula is CuSO₄·5H₂O. ✓

What is the percentage water content of a hydrated salt?

The percentage water content tells you what fraction of the total mass of the hydrated salt is water.

Formula:

% water = (mass of water of crystallisation / molar mass of hydrated salt) × 100

Worked example: calculate the percentage water content of CuSO₄·5H₂O.

• Molar mass of CuSO₄ = 160 g/mol

• Molar mass of 5H₂O = 5 × 18 = 90 g/mol

• Molar mass of CuSO₄·5H₂O = 160 + 90 = 250 g/mol

• % water = (90 / 250) × 100 = 36%

This means 36% of the mass of blue copper sulphate crystals is water — quite significant, and why the mass drops noticeably on heating.

How is water of crystallisation tested in experiments?

Test 1 — Colour change with copper sulphate: Anhydrous copper(II) sulphate (white powder) turns blue when water is added. This confirms the presence of water.

Test 2 — Cobalt chloride paper: Paper soaked in cobalt chloride solution is dried (blue anhydrous CoCl₂). In the presence of water, it turns pink as the hydrated form is regenerated.

Test 3 — Mass change on heating: Heating a hydrated salt to constant mass removes the water of crystallisation, causing a measurable mass loss. This mass loss can be used to calculate the number of water molecules in the formula (as shown above).

Frequently asked questions

Why is water of crystallisation part of the formula?

The water molecules in a hydrated salt are not simply trapped — they are bonded into the regular crystal lattice structure. The exact number of water molecules per formula unit is fixed: copper(II) sulphate always crystallises with exactly five water molecules per formula unit (not four, not six). This fixed stoichiometry means the water is an integral part of the compound's composition, and so it is included in the formula.

What happens if you heat a hydrated salt too strongly?

If a hydrated salt is heated too gently or too briefly, not all of the water of crystallisation is removed and the result will give an inaccurate calculation. If heated too strongly (or for too long), some ionic compounds may decompose further — for example, copper(II) sulphate can decompose to copper(II) oxide (CuO, black) at very high temperatures. This is why experiments specify heating to "constant mass" rather than a fixed temperature: you stop heating once repeated weighings show no further mass change.

How does anhydrous copper sulphate test for water?

Anhydrous copper(II) sulphate is a white powder. When water is added (even in small quantities), it reacts to form the hydrated compound CuSO₄·5H₂O, turning blue. This colour change is clearly visible, making it a sensitive qualitative test for the presence of water. Note that it does not test for pure water — any liquid containing water will give a positive result.

Is the water of crystallisation the same as water of hydration?

Yes — the terms "water of crystallisation" and "water of hydration" are used interchangeably for water molecules that are part of a crystal structure. "Hydrated" describes a salt that contains this water; "anhydrous" describes the same salt without it. The prefix "anhydrous" comes from Greek, meaning "without water".

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