Properties of Ionic Compounds
Revise Properties of Ionic Compounds for GCSE Chemistry with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. Ionic compounds have high melting points and conduct electricity only when molten or dissolved, because the ions are then free to move.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
Ionic compounds form giant ionic lattices, in which oppositely charged ions are held together by strong electrostatic forces of attraction acting in all directions.
Because those forces act throughout the whole structure, a large amount of energy is needed to overcome them, so ionic compounds have high melting and boiling points.
They do not conduct electricity when solid, because the ions are held in fixed positions and cannot move. But when melted or dissolved in water, the ions become free to move and carry charge, so the compound does conduct. Explaining that difference is the most frequently examined point in this topic.
Revision notes
The lattice structure
Ionic compounds consist of a giant lattice of oppositely charged ions.
Strong electrostatic forces of attraction act in all directions between the ions. This structure extends throughout the whole substance.
High melting and boiling points
Because the electrostatic forces are strong and act throughout the lattice, a large amount of energy is required to overcome them.
The substance therefore has a high melting point and a high boiling point.
Conducting electricity
When solid, the ions are held in fixed positions and cannot move, so no charge can flow and the compound does not conduct.
When molten or dissolved in water, the ions are free to move, so they can carry charge and the compound conducts electricity.
Key points
- Ionic compounds form giant lattices.
- Strong electrostatic forces act in all directions.
- They have high melting and boiling points.
- They do not conduct when solid.
- Ions in a solid are held in fixed positions.
- They conduct when molten or dissolved because ions can move.
Worked examples
Example 1
Explain why ionic compounds have high melting points. [2 marks]
Model answer
They have a giant lattice structure with strong electrostatic forces of attraction between oppositely charged ions acting in all directions (1 mark), so a large amount of energy is needed to overcome these forces (1 mark).
Example 2
Explain why sodium chloride conducts electricity when molten but not when solid. [3 marks]
Model answer
When solid, the ions are held in fixed positions in the lattice and cannot move (1 mark), so no charge can flow. When molten, the ions are free to move (1 mark) and can therefore carry electrical charge through the liquid (1 mark).
Example 3
Explain why ionic compounds conduct electricity when dissolved in water. [2 marks]
Model answer
Dissolving separates the ions from the lattice (1 mark), so they are free to move through the solution and carry charge (1 mark).
Common mistakes
Saying electrons carry the charge in ionic compounds.
It is the ions that move and carry charge, not electrons.
Saying ionic compounds always conduct.
They only conduct when molten or dissolved, never when solid.
Forgetting that forces act in all directions.
That detail explains why the structure is so strong and often carries a mark.
Saying the ions are destroyed on melting.
They remain as ions; they simply become free to move.
Exam tips
- Say electrostatic forces in all directions when explaining melting points.
- Explain conduction through ions being free to move.
- Cover both the solid and the molten case when asked to compare.
- Never say electrons move in an ionic compound.
Key terms
- Giant ionic lattice
- A regular structure of many oppositely charged ions.
- Electrostatic force
- The attraction between oppositely charged ions.
- Molten
- Melted, so the ions are free to move.
- Aqueous
- Dissolved in water.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.