Properties of ionic compounds — AQA GCSE Combined Science
Test yourself on Properties of ionic compounds with AQA GCSE practice questions.
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Properties of ionic compounds explained
When a metal reacts with a non-metal, electrons are transferred to form positive and negative ions.
Read the full explanation
These ions do not sit in pairs; they pack into a regular three-dimensional arrangement called a giant ionic lattice. In sodium chloride, each Na⁺ ion is surrounded by six Cl⁻ ions and each Cl⁻ by six Na⁺ ions, so the pattern repeats in every direction. The lattice is held together by strong electrostatic forces of attraction between oppositely charged ions. These forces act in all directions, not just between one pair of ions, which is why the structure is described as giant. The regular arrangement also explains why ionic crystals have flat faces and cleave along definite planes. Understanding this structure is the foundation for explaining the high melting and boiling points of ionic compounds.
These compounds have high melting points and high boiling points because of the large amounts of energy needed to break the many strong bonds.
Ionic compounds such as sodium chloride have high melting points and high boiling points. This is because a giant ionic lattice contains many strong electrostatic forces of attraction between oppositely charged ions, and these forces act throughout the structure. Melting or boiling requires enough energy to overcome these forces so that the ions can move apart. Because there are many strong attractions in a giant lattice, a large amount of energy is needed, so the melting point and boiling point are high. For example, sodium chloride melts at about 801 °C. The energy is used to break the many electrostatic attractions, not to break the ions themselves. This links the structure of ionic compounds directly to their physical properties.
When melted or dissolved in water, ionic compounds conduct electricity because the ions are free to move and so charge can flow.
Ionic compounds form giant lattices of alternating positive and negative ions held by strong electrostatic attraction. In the solid state these ions vibrate about fixed positions, so although each ion carries charge, no ion can travel through the lattice and no current flows. Melting supplies enough energy to overcome the attractions, releasing the ions so they can slide past one another. Dissolving in water has a similar effect: water molecules pull ions away from the lattice and surround them, forming a solution in which the ions are mobile. In both cases the charged particles themselves move, so charge can flow and the compound conducts electricity. For example, solid sodium chloride does not light a bulb, but molten sodium chloride and sodium chloride solution both conduct because Na⁺ and Cl⁻ ions are free to move.
Your focus
- Describe the arrangement of ions in a giant ionic lattice.
- Explain that ionic lattices are held together by strong electrostatic forces of attraction acting in all directions.
- Use a named ionic compound to illustrate the regular repeating structure of a giant ionic lattice.
Show all 9 objectives
- Explain why ionic compounds have high melting points and high boiling points.
- Relate the energy needed to break many strong electrostatic forces to the giant ionic lattice structure.
- Use a named ionic compound to illustrate the link between structure and melting or boiling point.
- Describe the arrangement of ions in a solid ionic lattice and explain why the solid does not conduct electricity.
- Explain how melting or dissolving frees ions from the lattice so that charge can flow.
- Apply the idea of mobile ions to explain conduction in molten and aqueous ionic compounds, using sodium chloride as an example.
Properties of ionic compounds exam tips
Marking Points
- Ions are arranged in a regular repeating pattern, not randomly or in isolated pairs.
- The structure extends in three dimensions, so it is described as a giant lattice.
- Oppositely charged ions attract one another by electrostatic forces.
- The attraction acts in all directions around each ion, not only along one line.
- A named example such as sodium chloride can be used to show six-fold coordination of Na⁺ and Cl⁻.
- The forces are between ions, so they are electrostatic rather than covalent bonds between molecules.
- High melting and boiling points are caused by strong electrostatic forces between oppositely charged ions.
- The lattice is giant, so there are many strong attractions to overcome.
- A large amount of energy is needed to break these many strong attractions.
- Melting and boiling involve overcoming the forces between ions, not breaking the ions themselves.
- A named example, such as sodium chloride melting at about 801 °C, can support the explanation.
- The explanation should link structure to property rather than simply stating that ionic compounds have high melting points.
- Solid ionic compounds do not conduct because their ions are held in fixed positions in the giant lattice and cannot move through the structure.
- Melting provides energy to overcome the strong electrostatic attractions between oppositely charged ions, so the ions become free to move.
- When an ionic compound dissolves, water molecules separate the ions from the lattice and surround them, so the ions are free to move in solution.
- Conduction occurs because the moving ions carry charge through the liquid or solution, completing the circuit.
- The charge carriers in an ionic compound are the ions themselves, not electrons as in a metal.
- Both molten and aqueous ionic compounds conduct for the same underlying reason: mobile ions are present.
Examiner Tips
- 💡Use the phrase 'giant ionic lattice' and state that it is a regular arrangement.
- 💡When explaining properties, always link the structure back to electrostatic forces between oppositely charged ions.
- 💡If asked to draw a diagram, label the ions with their charges, for example Na⁺ and Cl⁻, and show the pattern repeating.
- 💡Always mention both the strength and the number of electrostatic forces when explaining high melting points.
- 💡Use the phrase 'large amounts of energy' rather than vague wording such as 'lots of power'.
- 💡Link the property to the giant lattice structure in the same sentence to show cause and effect.
- 💡Link the state of the substance to the mobility of its ions in every explanation: solid means fixed ions, molten or aqueous means mobile ions.
- 💡Use the phrase free to move and so charge can flow when explaining conduction, and name the charge carriers as ions.
- 💡Compare a solid sample with the same compound when molten or dissolved to show that the difference is ion mobility, not a change in the ions themselves.
Common Mistakes
- Drawing only one Na⁺ ion next to one Cl⁻ ion and calling it a lattice; correction: show a repeating 3-D pattern with each ion surrounded by oppositely charged ions.
- Saying the ions are held by covalent bonds; correction: the attraction is electrostatic between oppositely charged ions, not shared electrons.
- Describing the forces as acting only between neighbouring pairs; correction: the electrostatic attraction acts in all directions throughout the lattice.
- Saying that covalent bonds between ions are broken; correction: the electrostatic forces of attraction between ions are overcome.
- Stating that only a few bonds need to be broken; correction: a giant lattice contains many strong attractions, so a large amount of energy is needed.
- Confusing melting with dissolving; correction: melting is the change from solid to liquid when the lattice is broken down by heating, not by a solvent.
- Saying that solid ionic compounds conduct because ions are charged. Correction: charge alone is not enough; the ions must be free to move, and in a solid lattice they are not.
- Stating that electrons flow through molten or dissolved ionic compounds. Correction: the mobile charge carriers are ions, so the current is carried by moving positive and negative ions.
- Claiming that melting destroys the ions or that dissolving breaks ions into atoms. Correction: the ions remain intact; melting and dissolving only free them from the lattice so they can move.