AQA GCSE Chemistry Foundation (8462), Foundation tier · Bonding, structure and the properties of matter › Bonding, structure and properties
Practise Properties of small molecules. 7 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 substances made of small molecules have low melting and boiling points and do not conduct electricity. The key idea, tested again and again, is that melting and boiling overcome the weak intermolecular forces between molecules, not the strong covalent bonds inside them.
Key facts
Small molecules: strong covalent bonds inside, weak intermolecular forces between
Melting and boiling overcome the intermolecular forces, NOT the covalent bonds
Weak forces → little energy needed → low melting and boiling points
Molecules have no overall charge → do not conduct electricity
Notes
Bonds inside, forces between
diagram
Small molecules (e.g. H2, O2, Cl2, H2O, CO2, CH4) have strong covalent bonds between the atoms inside each molecule.
Between separate molecules there are only weak intermolecular forces.
When a molecular substance melts or boils, it is the intermolecular forces that are overcome. The covalent bonds do not break: the molecules stay the same.
Melting or boiling overcomes the weak forces (dashed), not the covalent bonds.
Low melting and boiling points
Weak intermolecular forces need only a small amount of energy to overcome.
So substances made of small molecules have low melting and boiling points. They are usually gases or liquids at room temperature.
The intermolecular forces increase with the size of the molecules, so larger molecules have higher melting and boiling points.
Example: in Group 7, fluorine and chlorine are gases, bromine is a liquid and iodine is a solid at room temperature, because the molecules get larger down the group.
Example: methane, CH4, boils at −162 °C, but pentane, C5H12, which has larger molecules, boils at 36 °C.
No electrical conductivity
Molecules have no overall electric charge, and there are no ions or delocalised electrons free to move.
So substances made of small molecules do not conduct electricity, whether they are solid, liquid or gas.
How to answer each type of question
Explain why a molecular substance does not conduct electricity
1 to 2 marksGrade 4
Say the molecules have no overall electric charge.
So there are no charged particles free to move and carry charge.
Example. Explain why liquid bromine does not conduct electricity.
Show the model answerHide the model answer
Bromine is made of molecules that have no overall electric charge (1), so there are no charged particles free to move and carry charge (1).
Explain why a molecular substance has a low boiling point
2 to 3 marksGrade 5
Say it is made of small molecules.
Say there are weak intermolecular forces between the molecules.
Say only a small amount of energy is needed to overcome them.
Example. Chlorine, Cl2, boils at −34 °C. Explain why chlorine has a low boiling point.
Show the model answerHide the model answer
Chlorine is made of small molecules (1). There are weak intermolecular forces between the molecules (1). Only a small amount of energy is needed to overcome these forces (1).
Don’t lose marks
Saying covalent bonds break when a molecular substance melts or boils. This is the most common mistake in the topic.
Writing 'weak covalent bonds'. Covalent bonds are strong; it is the intermolecular forces that are weak.
Writing 'weak bonds' or 'weak forces' without saying they are between the molecules.
Saying the molecules themselves melt or expand when heated.
Explaining a trend with 'more bonds' instead of 'stronger intermolecular forces between larger molecules'.
More tips
Memory tricks
Inside strong, between weak: covalent bonds hold each molecule together; intermolecular forces hold the molecules to each other.
Low melting point and never conducts → small molecules.
Bigger molecule, bigger boiling point.
Exam technique
Always write 'intermolecular forces' or 'forces between molecules'. A vague 'bonds' usually loses the mark.
If you compare with a giant covalent substance, say that its covalent bonds must be broken to melt it.
Quote the data when you describe a trend.
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 small molecules have low boiling pointsSubstances made of small molecules are usually gases or liquids at room temperature.
Grade 4
State that small molecules don't conduct electricityThe molecules have no overall electric charge.
Grade 5
Explain low boiling points using intermolecular forcesOnly weak intermolecular forces must be overcome, which needs little energy.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Explain why substances that consist of small molecules do not conduct electricity.
The molecules do not have an overall electric charge.
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
Many covalent substances consist of small molecules.
(a) Complete the sentence. Tick (✓) one box. Substances that consist of small molecules are usually ................[1]
gases or liquids
hard solids
metals
good conductors of electricity
(b) What is overcome when water boils? Tick (✓) one box.[1]
Covalent bonds
Intermolecular forces
Ionic bonds
Metallic bonds
(c) Explain why substances that consist of small molecules do not conduct electricity.[1]
(d) Methane boils at −162 °C. What is the state of methane at room temperature?[1]
Show the answer and mark scheme
(a)Answer: gases or liquids
(b)Answer: Intermolecular forces
(c)Answer: The molecules do not have an overall electric charge.
the molecules have no overall (electric) charge
(d)Answer: gas
gas
Question 2Medium5 marks
To boil one mole of water needs 41 kJ of energy. To break all of the O–H covalent bonds in one mole of water molecules needs 928 kJ of energy.
(a) Calculate how many times more energy is needed to break the covalent bonds in one mole of water than to boil one mole of water.[2]
(b) A student said: ‘When water boils, the covalent bonds break and the molecules split up into hydrogen and oxygen.’ Use the data to explain why the student must be wrong.[1]
(c) Describe what happens to the water molecules when water boils.[2]
Show the answer and mark scheme
(a)Answer: 23 (22.6)
928 ÷ 41
22.6 / 23
(b)Answer: If the covalent bonds broke, about 928 kJ would be needed to boil one mole of water, but only 41 kJ is needed.
breaking the covalent bonds would need 928 kJ per mole, but boiling needs only 41 kJ per mole / boiling needs far less energy than breaking the bonds
(c)Answer: The weak intermolecular forces between the water molecules are overcome, and the molecules move far apart and move randomly as a gas (steam).
the (weak) intermolecular forces / forces between the molecules are overcome
the molecules move far apart / move randomly and quickly as a gas