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Water of crystallisation: what is it? calculations & determination

5. GCSE level chemistry notes on explaining water of crystallisation in salt crystals, what is it? calculations? determination?

[Author © Dr WP Brown PhD: Doc Brown's chemistry exam revision notes suitable for students of UK IGCSE & GCSE level chemistry courses & ~ US grades 9-10 chemistry, updated Feb 2nd 2026 *]

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The explanation and calculation of water of crystallisation

Water chemistry notes index

5. Water of crystallisation calculations

  • See also chemical calculations

  • Part 14.4 for more examples and questions on water of crystallisation

  • What is water of crystallisation?

    • Water of crystallization are the molecules of water that are incorporated into some salt crystals when they are crystallised out of water.

    • e.g. when blue crystals of copper(II) sulfate are crystallized out of water the actual formula of the crystals is ...

    • NOT simply CuSO4, but on crystallisation CuSO4.5H2O is formed,

    • because five water molecules are associated with each 'CuSO4'. in its 'hydrated' crystalline form.

    • -

  • Solubility graphs and data are covered in section 4.

    • -

  • How to calculate the theoretical % of water in a hydrated salt

    • eg magnesium sulphate MgSO4.7H2O salt crystals

    • Relative atomic masses: Mg = 24, S = 32, O = 16 and H = 1

    • Relative formula mass of water = (2 x 1) + 16 = 18

    • Relative formula mass of MgSO4 = 24 + 32 + (4 x 16) = 120

    • Relative mass of seven water molecules = 7 x 18 = 126

    • Relative formula mass of crystals = MgSO4 + (7 x H2O) = 120 + 126 = 246

    • so % water = 126 x 100 / 246 = 51.2%

    • -

  • Determination and calculation of salt formula containing 'water of crystallisation'.

    • Some salts, when crystallised from aqueous solution, incorporate water molecules into the structure. This is known as 'water of crystallisation', and the 'hydrated' form of the compound.

    • e.g. magnesium sulphate MgSO4.7H2O. The formula can be determined by a simple experiment (see the copper sulphate example below).

    • A known mass of the hydrated salt is gently heated in a crucible until no further water is driven off and the weight remains constant despite further heating. The mass of the anhydrous salt left is measured. The original mass of hydrated salt and the mass of the anhydrous salt residue can be worked out from the various weighings.

    • The % water of crystallisation and the formula of the salt are calculated as follows:

      • Suppose 6.25g of blue hydrated copper(II) sulphate, CuSO4.xH2O, (x unknown) was gently heated in a crucible until the mass remaining was 4.00g. This is the white anhydrous copper(II) sulphate.

      • The mass of anhydrous salt = 4.00g, mass of water (of crystallisation) driven off = 6.25-4.00 = 2.25g

      • The % water of crystallisation in the crystals  is 2.25 x 100 / 6.25 = 36%

      • [ Ar's Cu=64, S=32, O=16, H=1 ]

      • The mass ratio of CuSO4 : H2O is 4.00 : 2.25

      • To convert from mass ratio to mole ratio, you divide by the molecular mass of each 'species'

      • CuSO4 = 64 + 32 + (4x18) = 160 and H2O = 1+1+16 = 18

      • The mole ratio of CuSO4 : H2O is 4.00/160 : 2.25/18

      • which is 0.025 : 0.125 or 1 : 5,

      • so the formula of the hydrated salt is CuSO4.5H2O

      • (Note the more accurate atomic mass of copper is 63.5)

      • -

  • See also chemical calculations

  • Part 14.4 for more examples and questions on water of crystallisation

    • -

  • All concentration calculations are covered on the on-line CLICK for GCSE Chemical Calculations calculations page, especially sections 7. on molarity, 11. and 12. on molarity and acid-base (alkali) titrations, section 14.3 on dilutions.

    • -

  • Advanced level pre-university chemistry notes on the structure of hydrated salts

    • -

Practice questions with worked out answers

See also Chemical calculations part 14.4

14.4 water of crystallisation calculations

Key points: Chemistry - Water of crystallisation, explanation, determination & calculations

The key points outlined below were obtained by using AI internet searches analysing specifications for most exam boards offering UK GCSE & IGCSE science-chemistry courses

A syllabus-aligned summary of water of crystallisation for GCSE/IGCSE Chemistry, tailored to WJEC, CCEA, CIE, AQA, Edexcel, OCR Gateway, and OCR 21st Century specifications. It includes definitions, examples, calculations, tips, and common misconceptions to support student understanding and exam success.

What Is Water of Crystallisation?

Term Definition Example
Hydrated Salt A salt that contains water molecules in its crystal structure CuSO4.5H2O (blue crystals)
Anhydrous Salt A salt that has had its water of crystallisation removed CuSO4 (white powder)
Water of Crystallisation Water molecules chemically bonded within a salt’s crystal lattice The “·5H2O” in CuSO4·5H2O
  • These water molecules are not just trapped   -  they are chemically bonded and affect the salt’s colour, mass, and reactivity.

How to Determine Water of Crystallisation

Experimental Method:

  1. Weigh a sample of hydrated salt.
  2. Heat gently to remove water.
  3. Reweigh the anhydrous salt.
  4. Repeat heating until mass remains constant.
  5. Use mass loss to calculate number of water molecules.

Example:

  • Mass of hydrated salt = 6.10 g
  • Mass after heating = 5.20 g
  • Mass of water lost = 0.90 g

How to Calculate Water of Crystallisation

Steps:

  1. Calculate moles of anhydrous salt:
    • Moles = mass / molar mass
  2. Calculate moles of water lost:
    • Moles = mass / 18
  3. Find ratio of salt : water
  4. Round to nearest whole number → gives x in formula: Salt.xH2O

Worked Example:

  • Hydrated salt: AB2.xH2O (assume AB2 has a formula mass of 208)
  • Mass of hydrated salt = 6.10 g
  • Mass of anhydrous AB2 = 5.20 g
  • Mass of water = 0.90 g
  • Moles of AB2 = 5.20 ÷ 208 = 0.025 mol
  • Moles of H2O = 0.90 ÷ 18 = 0.05 mol
  • Ratio = 1 : 2 → x = 2
  • Final formula: AB2.2H2O

Student Tips about water of crystallisation

  • Always heat to constant mass to ensure all water is removed.
  • Use accurate molar masses and round ratios carefully.
  • Know these are common hydrated salts you will encounter:
    • CuSO4.5H2O (blue)
    • CoCl2.6H2O (pink)
    • MgSO4.7H2O (Epsom salts)
  • Practice exam-style calculations and formula writing.

Common Misconceptions about water of crystallisation

Misconception Correction
Water of crystallisation is just trapped water It is chemically bonded in the crystal lattice
Hydrated salts are wet They contain water in structure, not as liquid
Heating once removes all water Must heat until mass is constant
Formula is CuSO4·H2O It’s CuSO2.5H2O   -  five water molecules per unit
Mass loss is due to decomposition It’s due to loss of water, not breakdown of salt

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Extra Aqueous Chemistry Index:

1. Water cycle, treatment, pollution

2. Colloids - sols, foam and emulsions

3. Hard and soft water - causes and treatment

4. Gas and salt solubility in water and solubility curves

5. Calculation of water of crystallisation (this page)

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