AQA GCSE Chemistry Foundation (8462), Foundation tier · Bonding, structure and the properties of matter › Bonding, structure and properties
Practise Properties of ionic compounds. 8 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
Why ionic compounds have high melting and boiling points, and why they conduct electricity only when melted or dissolved in water. These are classic 2 to 4 mark 'explain' questions, and they often appear alongside metals and covalent substances in comparison questions.
Key facts
Ionic compounds: high melting and boiling points; solids at room temperature
Reason: many strong electrostatic forces between oppositely charged ions → lots of energy to overcome
Solid: does not conduct (ions fixed in place, can only vibrate)
Molten or dissolved: conducts (ions free to move and carry charge)
Ionic compounds conduct by moving ions, not electrons
Notes
High melting and boiling points
Ionic compounds are giant ionic lattices with strong electrostatic forces of attraction between oppositely charged ions, acting in all directions.
To melt or boil the compound, these forces must be overcome. There are a very large number of strong forces, so a large amount of energy is needed.
So ionic compounds have high melting and boiling points and are solids at room temperature. For example, sodium chloride melts at 801 °C.
Conducting electricity
diagram
To conduct electricity, a substance must contain charged particles that are free to move.
In a solid ionic compound the ions are held in fixed positions in the lattice. They can only vibrate, so the solid does not conduct.
When the compound is melted or dissolved in water, the ions are free to move, so they can carry charge through the liquid or solution: it conducts.
Ions can only carry charge when they are free to move.
In ionic compounds it is the ions that move and carry the charge, not electrons. This is the basis of electrolysis.
Using the ideas
A substance with a high melting point that conducts only when molten or dissolved is likely to be ionic.
Compare with metals, which conduct when solid because their delocalised electrons can move, and with small molecules, which never conduct.
How to answer each type of question
Explain why an ionic compound has a high melting point
3 marksGrade 5
Name the structure: a giant ionic lattice.
Name the forces: strong electrostatic forces of attraction between oppositely charged ions.
Link: a lot of energy is needed to overcome these forces.
Example. Potassium chloride has a melting point of 770 °C. Explain why potassium chloride has a high melting point.
Show the model answerHide the model answer
It has a giant ionic lattice (1). There are strong electrostatic forces of attraction between the oppositely charged K+ and Cl− ions (1). A large amount of energy is needed to overcome these forces (1).
Identify an ionic compound from data
2 to 3 marksGrade 5
Look for a high melting point.
Look for 'conducts when molten or dissolved, but not when solid'.
Give the answer and quote the evidence.
Example. The table gives information about three substances.
Substance
Melting point in °C
Conducts when solid?
Conducts when liquid (molten)?
P
993
no
yes
Q
1085
yes
yes
R
−95
no
no
Which substance is ionic? Give two reasons for your answer.Show the model answerHide the model answer
P (1). It has a high melting point (1). It conducts when molten but not when solid (1).
Don’t lose marks
Saying ionic compounds conduct because of free or delocalised electrons. It is the ions that move.
Saying the ions are free to move in the solid, or that solid ionic compounds conduct.
Writing only 'strong bonds'. Say what they are: electrostatic forces of attraction between oppositely charged ions.
Mentioning intermolecular forces for an ionic compound. There are no molecules.
Saying 'it takes a lot of energy to break' without saying what is being overcome.
More tips
Memory tricks
Ionic conduction: solid, no; molten or aqueous, yes. The ions have to be free to move.
Three-step melting point answer: structure → forces → energy.
Ask 'what is moving?': electrons in metals, ions in molten or dissolved ionic compounds.
Exam technique
Learn the full chain for high melting points: giant lattice, strong electrostatic forces between oppositely charged ions, lots of energy needed to overcome them.
If asked why it conducts when molten but not when solid, cover both states.
Say 'free to move', not just 'move': the key idea is that the ions can move through the substance.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
State that ionic compounds have high melting pointsIonic compounds such as sodium chloride are solids at room temperature with high melting and boiling points.
Grade 4
State when ionic compounds conduct electricityWhen melted (molten) or dissolved in water, but not when solid.
Grade 5
Explain high melting points using electrostatic forcesA lot of energy is needed to overcome the many strong electrostatic forces between oppositely charged ions.
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
Sodium chloride is an ionic compound.
(a) Why does sodium chloride have a high melting point? Tick (✓) one box.[1]
Large amounts of energy are needed to break the many strong bonds.
Its molecules are very large.
The intermolecular forces are weak.
The ions are free to move.
(b) When does sodium chloride conduct electricity? Tick (✓) two boxes.[2]
When it is solid
When it is molten
When it is dissolved in water
Never
(c) Explain why solid sodium chloride does not conduct electricity.[1]
Show the answer and mark scheme
(a)Answer: Large amounts of energy are needed to break the many strong bonds.
(b)Answer: When it is molten; When it is dissolved in water
(c)Answer: The ions are held in fixed positions, so they cannot move to carry charge.
the ions are in fixed positions / cannot move (to carry charge)
Question 2Medium6 marks
Magnesium oxide melts at 2852 °C. Sodium chloride melts at 801 °C.
(a) Explain why magnesium oxide has a high melting point.[3]
(b) Suggest why magnesium oxide has a higher melting point than sodium chloride.[2]
(c) Magnesium oxide is used to line the inside of furnaces. Suggest why.[1]
Show the answer and mark scheme
(a)Answer: It is a giant ionic lattice with strong electrostatic forces between oppositely charged ions; lots of energy is needed to overcome them.
giant (ionic) lattice / structure
strong electrostatic forces of attraction between oppositely charged ions
a lot of energy is needed to overcome the forces / break the bonds
(b)Answer: Mg2+ and O2− ions have larger charges than Na+ and Cl− ions, so the electrostatic attraction is stronger.
the ions in magnesium oxide have larger charges (2+ and 2−) than those in sodium chloride (1+ and 1−)
so the electrostatic attraction between the ions is stronger
(c)Answer: It has a very high melting point, so it does not melt at furnace temperatures.
(very) high melting point / does not melt at high temperatures