Water of Crystallisation

Summary: Many salts crystallise with a fixed number of water molecules in their crystal lattice. Heating drives off this water, leaving the anhydrous salt. The formula M·xH₂O tells you how many water molecules are present per formula unit. Tags: igcse chemistry crystallisation water hydrated-salts calculations Created: 2026-07-14 Last Updated: 2026-07-14

Definitions

Common Hydrated Salts

Why Water of Crystallisation Matters

The water molecules in the crystal affect:

1. The Mr (relative formula mass) — the water contributes to the total mass
2. The colour of the compound — many salts change colour when dehydrated
3. Calculations involving masses of reactants/products — you need to account for the water

Heating to Remove Water of Crystallisation

Experimental Method

1. Weigh a clean, dry crucible (and lid) — record mass
2. Add the hydrated salt to the crucible; weigh again — find mass of hydrated salt
3. Heat strongly for several minutes (Bunsen burner, with lid partially on to prevent loss of solid but allow water vapour to escape)
4. Allow to cool in a desiccator (prevents reabsorption of moisture from air)
5. Weigh the crucible + anhydrous salt
6. Heat, cool, and re-weigh until constant mass is achieved (ensures ALL water is driven off)

“Heat to Constant Mass” — Why?

• You cannot tell just by looking whether all water has been removed
• Repeated heating → cooling → weighing ensures all water is gone
• When two consecutive mass readings are the same (within ±0.01 g), the salt is fully dehydrated

Calculations: Finding x in M·xH₂O

This is the key calculation skill tested in Paper 3, 4, and 5/6.

Method

1. Find mass of anhydrous salt (M) after heating to constant mass
2. Find mass of water lost = mass of hydrated salt − mass of anhydrous salt
3. Calculate moles of anhydrous salt = mass / Mr
4. Calculate moles of water = mass / 18
5. Find the simplest mole ratio = moles of water : moles of anhydrous salt
6. This gives x (round to nearest whole number)

Worked Example 1: Find x

3.75 g of hydrated sodium carbonate (Na₂CO₃·xH₂O) is heated to constant mass. The mass of anhydrous Na₂CO₃ remaining is 1.36 g. Find x.

1. Mass of H₂O lost = 3.75 − 1.36 = 2.39 g
2. Moles of anhydrous Na₂CO₃ = 1.36 / 106 = 0.0128 mol
3. Moles of H₂O = 2.39 / 18 = 0.133 mol
4. Mole ratio H₂O : Na₂CO₃ = 0.133 : 0.0128 = 10.4 : 1
5. Round to nearest integer: x = 10
6. Formula = Na₂CO₃·10H₂O (sodium carbonate decahydrate / washing soda)

Worked Example 2: Find x

6.24 g of hydrated copper(II) sulfate (CuSO₄·xH₂O) is heated to constant mass. 3.99 g of anhydrous CuSO₄ remains. Find x.

1. Mass of H₂O lost = 6.24 − 3.99 = 2.25 g
2. Moles of anhydrous CuSO₄ = 3.99 / 159.5 = 0.0250 mol
3. Moles of H₂O = 2.25 / 18 = 0.125 mol
4. Ratio H₂O : CuSO₄ = 0.125 : 0.0250 = 5 : 1
5. x = 5
6. Formula = CuSO₄·5H₂O (copper(II) sulfate pentahydrate — the familiar blue crystals)

Calculating Theoretical Mass Loss

Example

What mass of water is lost when 5.00 g of MgSO₄·7H₂O is heated to constant mass?

1. Mr(MgSO₄·7H₂O) = 24 + 32 + (4×16) + 7(2+16) = 120 + 126 = 246
2. Moles of MgSO₄·7H₂O = 5.00 / 246 = 0.0203 mol
3. Each mole contains 7 moles of H₂O → moles of H₂O = 7 × 0.0203 = 0.142 mol
4. Mass of H₂O = 0.142 × 18 = 2.56 g
5. Mass of anhydrous MgSO₄ remaining = 5.00 − 2.56 = 2.44 g

Anhydrous Salts as Tests for Water

Copper(II) Sulfate Test

• Anhydrous CuSO₄ (white powder) + water → blue (hydrated CuSO₄·5H₂O forms)
• The colour change from white → blue indicates the presence of water

Cobalt Chloride Paper

• Anhydrous CoCl₂ (blue) + water → pink (hydrated CoCl₂·6H₂O forms)
• Cobalt chloride paper changes from blue → pink in the presence of water
• Used to test for water in a gas stream

Note: These tests detect water specifically, not just any liquid. They do NOT test for acidity or purity — they test for the presence of H₂O.

Common Question Types

Type 1: Find x in M·xH₂O from Experimental Data (3-4 marks)

• Frequency: ~35% of Papers 3/4; ~50% of Papers 5/6
• Most common format: mass before and after heating → find moles ratio → x
• Answer is usually a whole number

Type 2: Calculate Theoretical Percentage of Water (2-3 marks)

• % H₂O by mass = (mass of water of crystallisation / Mr of hydrated salt) × 100
• e.g., % H₂O in MgSO₄·7H₂O = (126/246) × 100 = 51.2%

Type 3: Predict Colour Change (1-2 marks)

• “What colour change would you observe when heating blue copper(II) sulfate crystals?”
• Blue → white (water is driven off)

Type 4: Explain “Heat to Constant Mass” (2 marks)

• To ensure ALL water of crystallisation has been removed
• Two consecutive weighings give the same result

Common Mistakes

• Using Mr of anhydrous salt when hydrated salt Mr is needed: Check the formula given — CuSO₄·5H₂O has Mr = 249.5, not 159.5
• Not rounding x to a whole number: The water of crystallisation must be an integer
• Poor precision in mole calculations: Rounding too early gives wrong x
• Forgetting that cooling must be in a desiccator: Anhydrous salts absorb moisture from air
• Heating too strongly: Some salts (e.g., CuSO₄) decompose at very high temperatures (→ CuO + SO₃), giving misleading mass loss
• Not heating to constant mass: Some water may remain, giving a partial x value (too low)

Key Facts to Memorize

• CuSO₄·5H₂O: blue → white on heating (anhydrous CuSO₄ is white)
• CoCl₂·6H₂O: pink → blue on heating (used in cobalt chloride test paper)
• Moles of H₂O lost = mass loss / 18
• Ratio H₂O : anhydrous salt = x (round to nearest integer)
• “Heat to constant mass” = heat → cool → weigh → repeat until mass doesn’t change
• Mr of hydrated salt = Mr(anhydrous) + (x × 18)

Related Notes

• Percentage Yield and Purity — Purity calculations involving hydrated salts
• Relative Masses and Moles — Mr calculations for hydrated compounds
• Reacting Masses — Reacting mass calculations considering water of crystallisation
• Salt Preparation Methods — Making hydrated salt crystals
• Chemical Tests — Anhydrous CuSO₄ and CoCl₂ paper as water tests
• Separation Techniques — Crystallisation technique
• Experimental Techniques — Heating to constant mass technique
• IGCSE-Chem-Index

Past Paper Sources

• 0620/43 May/June 2018 Q7(d): Calculate x in Na₂CO₃·xH₂O from heating data (4 marks)
• 0620/53 Oct/Nov 2019 Q2: Water of crystallisation experiment — method + find x (6 marks)
• 0971/42 Feb/March 2021 Q5(e): Calculate % of water in hydrated magnesium sulfate (3 marks)
• 0620/32 May/June 2020 Q6(c): Write formula of hydrated salt; predict colour change (2 marks)
• 0971/63 Oct/Nov 2022 Q3: Practical question — heating hydrated salt to constant mass (5 marks)

IGCSE Chemistry (0620/0971) wiki. Important calculation topic often paired with practical assessment.