Some compounds require water molecules to form crystals. Without water, they will not form crystals and exist in powder form. The water needed to form crystals is called water of crystallization.
- Hydrated compounds: crystalline substances that contain water of crystallization
- Anhydrous compounds: powdered substances which have lost water of crystallization
☝️ Blue crystal is hydrated copper sulfate and white powder is anhydrous copper sulfate ☝️
Chemical Equation for Water of Crystallization
Hydrated compounds contain a fixed number of water molecules and the chemical formula is written as salt·xH2O.
For example, 5 moles of water is required for every mole of copper sulfate to form a crystal, so the chemical formula of hydrated copper sulfate is CuSO4·H2O.
The chemical equation to form hydrated copper sulfate from anhydrous copper sulfate is:
CuSO4 + 5H2O → CuSO4·5H2O
Anhydrous copper sulfate can be obtained by heating hydrated copper sulfate:
CuSO4·5H2O → CuSO4 + 5H2O
For IGCSE Chemistry, it is important to know how to determine the water of crystallization!
Steps to determine the water of crystallization:
- Find the mass of the hydrated salt.
- Find the mass of the anhydrous salt after all the water has left it after heating.
- Calculate the mass of water that is lost. (mass of water = mass of hydrated salt - mass of anhydrous salt)
- Calculate the amount of anhydrous salt in moles. (mole = mass / relative formula mass)
- Calculate the amount of water lost in moles.
- Find the ratio of anhydrous salt to water in simplest whole numbers.
- Write the formula of hydrated compound.
👇 Let's try some examples from IGCSE Chemistry 👇
Example 1
A hydrated salt has the formula AB2·xH2O. When the hydrated salt is heated, this reaction occurs.
AB2·xH2O → AB2 + xH2O
A scientist heats a sample of the hydrated salt until all the water has been lost. She records the mass of the salt before and after heating.
Mass of hydrated salt = 6.1 g
Mass of salt after heating = 5.2 g
(a) Describe how the scientist could make sure that all the water has been lost.
- If there is water left in the salt, more water can be lost when heated, so the mass of salt will decrease.
- When all the water has been lost, the mass of salt will remain constant.
- To make sure that all the water has been lost, the scientist should heat the salt until the mass of salt does not change.
(b) Use the scientist’s results to find the value of x in AB2·xH2O. [Mr of AB2 = 208, Mr of H2O = 18]
- mass of water lost = 6.1 - 5.2 = 0.9 g
- amount of AB2 in moles = 5.2 / 208 = 0.025 mol
- amount of water in moles = 0.9 / 18 = 0.05 mol
- ratio of AB2 to water = 1 : 2
- x = 2
- formula of hydrated salt = AB2·2H2O
Example 2
The mineral rozenite contains crystals of hydrated iron(II) sulfate, FeSO4·xH2O. A student wants to find the value of x. She uses this apparatus to remove and collect the water of crystallisation from a sample of iron(II) sulfate crystals.
She uses this method.
- weigh empty tube A to find its mass
- place a sample of hydrated iron(II) sulfate crystals into tube A and reweigh
- heat tube A
- allow tube A to cool and reweigh
- repeat the process until the mass no longer changes
When iron(II) sulfate crystals are heated gently, they decompose according to this equation.
FeSO4·xH2O → FeSO4 + xH2O
These are the student’s results.
- mass of tube A = 11.96 g
- mass of tube A and FeSO4·xH2O = 17.56 g
- mass of tube A and contents after heating to constant mass = 15.76 g
(a) Calculate the mass of anhydrous FeSO4 formed after heating to constant mass
mass of anhydrous iron(II) sulfate = 15.76 - 11.96 = 3.80 g
(b) Calculate the mass of water collected in tube B after heating to constant mass.
mass of water = 17.56 - 15.76 = 1.80 g
(c) Find the chemical formula of hydrated iron(II) sulfate.
- Mr of FeSO4 = 56 + 32 + 16 x 4 = 152
- Mr of water = 18
- amount of FeSO4 in moles = 3.80 / 152 = 0.025
- amount of water in moles = 1.80 / 18 = 0.1
- ratio of FeSO4 to water = 1 : 4
- x = 4
- formula of hydrated iron(II) sulfate = FeSO4·4H2O