Water of Crystallisation

Water of crystallisation is when some compounds can form crystals which have water as part of their structure
A compound that contains water of crystallisation is called a hydrated compound
The water of crystallisation is separated from the main formula by a dot when writing the chemical formula of hydrated compounds
- E.g. hydrated copper(II) sulfate is CuSO₄∙5H₂O
A compound which doesn’t contain water of crystallisation is called an anhydrous compound
- E.g. anhydrous copper(II) sulfate is CuSO₄
A compound can be hydrated to different degrees
- E.g. cobalt(II) chloride can be hydrated by six or two water molecules
- CoCl₂ ∙6H₂O or CoCl₂ ∙2H₂O

The conversion of anhydrous compounds to hydrated compounds is reversible by heating the hydrated salt:

Hydrated: CuSO₄•5H₂O ⇌ CuSO₄ + 5H₂O :Anhydrous

The degree of hydration can be calculated from experimental results:
- The mass of the hydrated salt must be measured before heating
- The salt is then heated until it reaches a constant mass
- The two mass values can be used to calculate the number of moles of water in the hydrated salt - known as the water of crystallisation

Worked example

Calculating water of crystallisation

11.25 g of hydrated copper sulfate, CuSO₄.xH₂O, is heated until a constant mass of 7.19 g.

Calculate the formula of the hydrated copper(II) sulfate.

A_r (Cu) = 63.5 A_r (S) = 3. A_r (O) = 16 A_r (H) = 1

Answer:

1. Salt and water	CuSO₄	H₂O
2. Value	7.19	11.25 - 7.19 = 4.06
3. M_r	63.5 + 32 + (16 x 4) = 159.5	(1 x 2) + 16 = 18
4. Moles = $\frac{mass}{M_{r}}$	$\frac{7.19}{159.5}$ = 0.045	$\frac{4.06}{18}$ = 0.226
5. Salt : water ratio	$\frac{0.045}{0.045}$ = 1	$\frac{0.226}{0.045}$ = 5
6. Formula of hydrated salt	The formula is CuSO₄•5H₂O

Examiner Tip

A water of crystallisation calculation can be completed in a similar fashion to an empirical formula calculation

Instead of elements, you start with the salt and water
Instead of dividing by atomic masses, you divide by molecular / formula masses
The rest of the calculation works the same way as the empirical formula calculation

Water of Crystallisation (CIE AS Chemistry)

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1. Physical Chemistry

1.1 Atomic Structure

1.1.1 Particles in the Atom & Atomic Structure

1.1.2 Mass, Charge & Subatomic Particles

1.1.3 Determining Subatomic Structure

1.1.4 Atomic & Ionic Radius

1.1.5 Isotopes

1.1.6 Electronic Structure

1.1.7 Electron Subshells & Orbitals

1.1.8 Electron Configuration

1.1.9 Determining Electronic Configurations

1.1.10 Ionisation Energy

1.1.11 Ionisation Energy Trends

1.1.12 Ionisation Energies & Electronic Configurations

1.2 Atoms, Molecules & Stoichiometry

1.2.1 Relative Masses

1.2.2 The Mole & Avogadro Constant

1.2.3 Formulae

1.2.4 Balancing Equations

1.2.5 Empirical & Molecular Formulae

1.2.6 Water of Crystallisation

1.2.7 Reacting Masses & Volumes

1.3 Chemical Bonding

1.3.1 Electronegativity

1.3.2 Electronegativity Trends

1.3.3 Electronegativity & Bonding

1.3.4 Ionic Bonding

1.3.5 Ionic Bonding Examples

1.3.6 Metallic Bonding

1.3.7 Covalent Bonding

1.3.8 Coordinate Bonding

1.3.9 Hybridisation

1.3.10 Bond Energy & Length

1.3.11 Shapes of Molecules

1.3.12 Hydrogen Bonding

1.3.13 Polarity & Dipole Moments

1.3.14 Van der Waals' Forces

1.3.15 Inter & Intramolecular Forces

1.3.16 Dot and Cross Diagrams

1.4 States of Matter

1.4.1 Gas Pressure

1.4.2 Ideal Gas Law & Equation

1.4.3 Lattice Structures

1.4.4 Bonding & Structure

1.5 Chemical Energetics

1.5.1 Enthalpy Change, ΔH

1.5.2 Energy Level Diagrams

1.5.3 Standard Enthalpy Change

1.5.4 Enthalpy & Bond Energies

1.5.5 Hess’s Law

1.6 Electrochemistry

1.6.1 Oxidation Number Rules

1.6.2 Redox Reactions

1.6.3 Oxidising & Reducing Agents

1.7 Equilibria

1.7.1 Chemical Equilibria

1.7.2 Le Chatelier's Principle

1.7.3 Equilibrium Constant

1.7.4 Equilibrium Constant Calculations

1.7.5 Changes Affecting the Equilibrium Constant

1.7.6 Industrial Processes

1.7.7 Acids & Bases

1.7.8 Brønsted–Lowry Theory

1.7.9 Acid & Base Dissociation

1.7.10 pH Scale

1.7.11 Strength of Acids & Bases

1.7.12 pH Titration Curves

1.7.13 Indicators used in Titration

1.8 Reaction Kinetics

1.8.1 Collision Theory

1.8.2 Rate of Reaction

1.8.3 Activation Energy & Boltzmann Distribution Curves

1.8.4 Homogeneous & Heterogeneous Catalysts

2. Inorganic Chemistry

2.1 The Periodic Table: Chemical Periodicity