AQA GCSE Combined Science Foundation (8464), Foundation tier · Chemistry › Bonding, structure, and the properties of matter › How bonding and structure are related to the properties of
Practise Polymers. 6 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 polymers such as poly(ethene) are solids at room temperature, and how to recognise a polymer from a diagram. Questions are usually short: explain the state of a polymer in terms of forces, or pick out the polymer from diagrams of different structures.
Key facts
Polymer = very large molecule made of many repeating units
Strong covalent bonds between the atoms in each chain
Relatively strong intermolecular forces between chains → solid at room temperature
Diagram: repeating unit in brackets, bonds through the brackets, n outside
Poly(ethene) repeating unit: (–CH2–CH2–)n
Melting a polymer overcomes intermolecular forces, not covalent bonds
Notes
What a polymer is
Polymers have very large molecules: long chains that can contain thousands of atoms.
Each chain is made of many copies of the same small section, called the repeating unit.
The atoms in each chain are joined to each other by strong covalent bonds.
Example: poly(ethene), used for plastic bags and bottles, is a long chain of carbon atoms, each carbon also bonded to two hydrogen atoms.
Why polymers are solids
Between the polymer molecules there are intermolecular forces.
Because the molecules are so large, these intermolecular forces are relatively strong: much stronger than the forces between small molecules.
So more energy is needed to overcome them, and polymers are solids at room temperature.
When a polymer such as poly(ethene) melts, it is the intermolecular forces between the chains that are overcome; the covalent bonds in the chains do not break.
Recognising a polymer in a diagram
diagram
A polymer is usually drawn as its repeating unit inside brackets, with a small n after the brackets (n is a large number).
The bonds at each end of the repeating unit go through the brackets, showing that the chain carries on.
Poly(ethene): two carbon atoms joined by a single bond, each with two hydrogen atoms, inside brackets with n.
Poly(ethene): the chain carries on in both directions.
Compare: a small molecule is drawn complete, with no bonds going off it; a giant covalent structure is a network of bonds going off in all directions, with no brackets or n; an ionic structure shows + and − ions.
How to answer each type of question
Identify a polymer from diagrams
1 to 2 marksGrade 5
Look for brackets with n and bonds that go through the brackets: that is a polymer.
A complete small unit is a small molecule; a network of bonds in all directions is giant covalent; + and − ions are ionic.
Example. Four structures are described. A: a molecule of one oxygen atom bonded to two hydrogen atoms. B: two carbon atoms, each bonded to two hydrogen atoms, inside brackets, with bonds through the brackets and n after them. C: a regular lattice of + and − ions. D: carbon atoms each bonded to four other carbon atoms in a network that continues in all directions. (a) Which structure is a polymer? (b) Which structure is a giant covalent structure?
Show the model answerHide the model answer
(a) B (1) (b) D (1)
Don’t lose marks
Saying polymers are solid because of strong covalent bonds between the chains. The chains are held together by intermolecular forces; the covalent bonds are inside the chains.
Calling a polymer a giant covalent structure. Polymers are made of separate, very large molecules.
Drawing a repeating unit without the bonds through the brackets, or leaving out the n.
Calling the intermolecular forces in polymers 'strong' without a comparison. They are relatively strong compared with those between small molecules, but much weaker than covalent bonds.
More tips
Memory tricks
Poly means many: a polymer is many repeating units joined in a chain.
Spot it: brackets + n + bonds sticking out through the brackets = polymer.
Melting point ladder: small molecules (low) < polymers (solids, but melt fairly easily) < giant covalent (very high).
Exam technique
Use comparisons: 'the molecules are larger, so the intermolecular forces are stronger'. Examiners want the link between size and the strength of the forces.
In 'which diagram' questions, check every option: usually one is a small molecule, one ionic and one giant covalent.
Name the right forces: intermolecular forces between polymer chains, covalent bonds within them.
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 polymers have very large moleculesPolymer molecules are long chains made of many repeating units.
Grade 4
State that polymers are solids at room temperatureThe forces between their large molecules are strong enough to keep them solid.
Grade 5
Describe the bonds in a polymer chainThe atoms in each chain are linked to each other by strong covalent bonds.
Grade 5
Recognise a polymer from a diagramLook for a repeating unit in brackets with bonds through the brackets and n after them.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
What name is given to the small molecules that join together to make a polymer?
monomers
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
Poly(ethene) is a polymer.
(a) Complete the sentence. Tick (✓) one box. Polymers have ................ molecules.[1]
very large
very small
charged
single-atom
(b) What type of bond joins the atoms within a polymer molecule? Tick (✓) one box.[1]
Covalent
Ionic
Metallic
Intermolecular
(c) Why is poly(ethene) a solid at room temperature? Tick (✓) one box.[1]
The intermolecular forces between the polymer molecules are relatively strong.
The polymer molecules are held together by ionic bonds.
Poly(ethene) contains delocalised electrons.
Poly(ethene) molecules are very small.
(d) What name is given to the small molecules that join together to make a polymer?[1]
Show the answer and mark scheme
(a)Answer: very large
(b)Answer: Covalent
(c)Answer: The intermolecular forces between the polymer molecules are relatively strong.
(d)Answer: monomers
monomers
Question 2Medium2 marks
A student says: ‘Poly(ethene) is a solid at room temperature because its covalent bonds are very strong.’
Evaluate the student’s statement.[2]
Show the answer and mark scheme
Answer: The covalent bonds in the chains are strong, but they are not broken when poly(ethene) melts. Poly(ethene) is a solid because its molecules are very large, so the intermolecular forces between them are relatively strong.
the covalent bonds within the chains are strong, but they are not broken when poly(ethene) melts / softens
it is a solid because the intermolecular forces between the (very large) polymer molecules are relatively strong