AQA GCSE Combined Science (8464), Higher tier · Chemistry › Bonding, structure, and the properties of matter › How bonding and structure are related to the properties of
Practise Properties of ionic compounds. 14 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.
Grade by grade
What you need to be able to do, from the first marks up to the top 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.
4
State when ionic compounds conduct electricityWhen melted (molten) or dissolved in water, but not when solid.
5
Explain high melting points using electrostatic forcesA lot of energy is needed to overcome the many strong electrostatic forces between oppositely charged ions.
6
Explain conduction in terms of moving ionsWhen molten or dissolved, the ions are free to move and carry charge; in the solid they are held in fixed positions.
8
Compare melting points using the charges on ionsIons with bigger charges (e.g. Mg2+ and O2−) attract each other more strongly, so more energy is needed to separate them.
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.
The bigger the charges on the ions, the stronger the attraction between them. Magnesium oxide (Mg2+ and O2− ions) has a much higher melting point than sodium chloride (Na+ and Cl− ions). grade 8+
Conducting electricity
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.
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.
Cheatsheet
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
Bigger charges on the ions → stronger attraction → higher melting point (MgO higher than NaCl) grade 8+
How to answer each type of question
Explain why an ionic compound has a high melting point
3 marks5
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 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 marks5
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. Substance P: melting point 993 °C; conducts when solid: no; conducts when molten: yes. Substance Q: melting point 1085 °C; conducts when solid: yes; conducts when molten: yes. Substance R: melting point −95 °C; conducts when solid: no; conducts when liquid: no. Which substance is ionic? Give two reasons for your answer.
Show the model answer
P (1). It has a high melting point (1). It conducts when molten but not when solid (1).
Explain when an ionic compound conducts electricity
3 marks6
Solid: the ions are in fixed positions, so they cannot move.
Molten or dissolved: the ions are free to move.
So the ions can carry charge through the liquid or solution.
Example. Solid sodium chloride does not conduct electricity, but sodium chloride solution does. Explain why.
Show the model answer
In the solid, the ions are held in fixed positions in the lattice, so they cannot move (1). In the solution, the ions are free to move (1) and carry charge through the solution (1).
Compare the melting points of two ionic compounds
3 marks8
Compare the charges on the ions in each compound.
Bigger charges mean stronger electrostatic forces of attraction.
Stronger forces need more energy to overcome.
Example. Magnesium oxide melts at about 2800 °C. Sodium chloride melts at 801 °C. Explain the difference in melting points.
Show the model answer
Magnesium ions (2+) and oxide ions (2−) have bigger charges than sodium ions (1+) and chloride ions (1−) (1). So the electrostatic forces of attraction between the ions in magnesium oxide are stronger (1). More energy is needed to overcome them (1).
Shortcuts and 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.
Where marks are lost
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.
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.
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
Question 3Hard6 marks
A technician has three unlabelled white solids. One is sodium chloride, one is glucose and one is silicon dioxide.
Plan an investigation to identify each solid. Your plan should explain the expected results in terms of the structure and bonding of each substance.[6]
Show the answer and mark scheme
Answer: Add each solid to water: silicon dioxide does not dissolve. Test the conductivity of the other two solutions: sodium chloride solution conducts (free ions), glucose solution does not (neutral molecules).
add the same mass of each solid to the same volume of water and stir
silicon dioxide does not dissolve (it is a giant covalent structure)
test the electrical conductivity of each solution using electrodes connected to a power supply and a bulb / ammeter
sodium chloride solution conducts because its ions are free to move and carry charge
glucose solution does not conduct because glucose is made of molecules with no overall charge
the solids themselves would not conduct, because the ions in solid sodium chloride are held in fixed positions
alternatively, heat small samples gently: glucose melts at a low temperature (weak forces between molecules); sodium chloride and silicon dioxide do not melt (strong bonds, high melting points)
safety: wear eye protection / switch off the power before handling the electrodes
Marked with levels of response: the full level descriptors are in the app.