Properties of Ionic CompoundsOxford AQA IGCSE Chemistry: Revision notes
Section 1
Why do ionic compounds have high melting and boiling points?
Ionic compounds have high melting and boiling points because of the strong electrostatic forces of attraction between oppositely charged ions, acting in all directions throughout the giant lattice.
To melt or boil an ionic compound, a very large number of these strong forces must be overcome, which requires a large amount of energy.
In exam answers, link 'high melting point' directly to 'strong electrostatic forces in all directions requiring large amounts of energy to overcome' — a one-word answer like 'strong bonds' will not get full marks.
Section 2
Do ionic compounds conduct electricity as solids?
No. In the solid state, the ions in an ionic compound are held tightly in fixed positions in the lattice by strong electrostatic forces. Because the ions cannot move, they cannot carry an electric current, so solid ionic compounds do not conduct electricity.
A common mistake is saying ionic solids don't conduct because 'ions have no charge' — ions definitely are charged; the real reason is that they cannot move in the solid lattice.
Section 3
When do ionic compounds conduct electricity?
Ionic compounds do conduct electricity when:
- Molten (melted) — the lattice breaks down and the ions are free to move
- Dissolved in water (aqueous) — the ions separate and are free to move through the solution
In both cases, the ions are free to move and carry the electric current through the liquid.
Molten sodium chloride conducts electricity in electrolysis because the Na⁺ and Cl⁻ ions are free to move to the electrodes and carry charge.
Must Know
- Ionic compounds have high melting and boiling points due to strong electrostatic forces acting in all directions, which need lots of energy to break
- Ionic compounds do not conduct electricity as solids — ions are fixed in place and cannot move
- Ionic compounds do conduct electricity when molten or dissolved in water, because the ions are then free to move and carry charge
- The key idea throughout is: ions must be free to move for conduction to happen
That's the notes covered.
Carry on to the next subtopic.