Revision notes for AQA GCSE Chemistry Polymers. Open the guide for explanations and worked examples. Written against the AQA GCSE Chemistry (8462) specification, so the content matches what's examinable rather than general Chemistry background.
Polymers
What you'll learn
- What a polymer is and why polymer molecules are so large.
- The difference between strong covalent bonds inside a polymer molecule and intermolecular forces between polymer molecules.
- Why polymers are usually solids at room temperature.
- How to recognise polymers from diagrams of their bonding and structure.
Big picture: structure controls properties
In this part of GCSE Chemistry, you are linking a substance’s structure to its properties. For polymers, the key idea is that they have molecules — but the molecules are very large compared with simple molecular substances like water, methane, or carbon dioxide.
Many everyday plastics are polymers, such as poly(ethene), poly(propene), and poly(chloroethene). You do not need to memorise lots of polymer names for this section. The main skill is recognising the structure and explaining the property.
Key Idea
The central idea
A polymer is made of very large molecules. The atoms within each polymer molecule are joined by strong covalent bonds, and the forces between different polymer molecules are relatively strong, so polymers are solids at room temperature.
Prerequisite: covalent bonds
A covalent bond is a strong bond formed when atoms share a pair of electrons. Covalent bonds usually form between non-metal atoms.
Definition
Covalent bond
A covalent bond is a shared pair of electrons between two atoms. It strongly holds atoms together inside a molecule.
In a polymer, the atoms in each long chain are joined by covalent bonds. For example, in a carbon-based polymer, carbon atoms may be joined to other carbon atoms, and to hydrogen atoms, by covalent bonds.
The important exam point is: covalent bonds act within a polymer molecule.
What is a polymer?
Definition
Polymer
A polymer is a substance made from very large molecules formed by many repeating units joined together.
A molecule is a group of atoms joined together by covalent bonds. A polymer molecule is still a molecule — it is just a very large one.
A small molecule might contain only a few atoms. A polymer molecule can contain thousands of atoms in a long chain.
The repeating section of a polymer chain is called the repeat unit. In diagrams, you may see brackets and a small nnn, such as [−CH2−CH2−]n\left[-\text{CH}_2-\text{CH}_2-\right]_n[−CH2−CH2−]n. The nnn means “many repeating units”.
Definition
Repeat unit
A repeat unit is the section of a polymer chain that repeats many times to make the full polymer molecule.
Polymers are not small simple molecules
Simple molecular substances, such as oxygen, water, and carbon dioxide, have small molecules. They often have low melting and boiling points because the forces between their molecules are weak.
Polymers are different because their molecules are very long. Even though each individual intermolecular force may not be as strong as a covalent bond, there are many contact points between long polymer chains.
Analogy
Long chains have lots of contact
Imagine two short pieces of thread lying next to each other: they separate easily. Two long tangled pieces of thread are much harder to pull apart. Polymer molecules behave a bit like long chains with many points of attraction between them.
Intermolecular forces between polymer molecules
An intermolecular force is a force of attraction between separate molecules.
Definition
Intermolecular force
An intermolecular force is an attraction between molecules. It is not a covalent bond and does not join atoms together inside a molecule.
In polymers, the intermolecular forces are described as relatively strong. This means they are stronger than the intermolecular forces in many small simple molecular substances.
They are not stronger than covalent bonds. Covalent bonds are still much stronger.
Common Mistake
Mixing up bonds and forces
Do not say polymers are solid at room temperature because “the covalent bonds between polymer molecules are strong”. The covalent bonds are within each polymer molecule. The forces between separate polymer molecules are intermolecular forces.
Why polymers are solids at room temperature
At room temperature, most polymers are solids because the relatively strong intermolecular forces between their large molecules need a lot of energy to overcome.
The long polymer chains sit close to each other. Because they are so long, there are many attractions between neighbouring chains. Altogether, these attractions make it difficult for the molecules to move freely past each other.
Example
Explaining why a polymer is solid
A student is asked: “Explain why poly(ethene) is a solid at room temperature.”
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Identify the structure: poly(ethene) is a polymer, so it contains very large molecules made from many repeating units.
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Link the structure to forces: the long polymer molecules have relatively strong intermolecular forces between them because there are many points of attraction between the chains.
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Link the forces to the property: at room temperature, there is not enough energy to overcome these forces, so the polymer molecules cannot move freely past each other. Therefore, poly(ethene) is a solid.
Recognising polymers from diagrams
In the exam, you may be shown diagrams of different substances and asked which one is a polymer.
Look for these features:
- A long chain of atoms, often mostly carbon atoms.
- A repeating pattern along the chain.
- Atoms joined by covalent bonds within the chain.
- Sometimes brackets around a repeat unit, with an nnn outside the brackets.
A polymer diagram may show only part of the chain. Dotted or broken ends can mean the chain continues further.
Tip
How to spot a polymer quickly
If the diagram shows a covalent molecule that keeps repeating the same small section again and again, it is probably a polymer.
Example
Recognising a polymer from a diagram
A diagram shows a long chain of carbon atoms. Each carbon atom is covalently bonded to hydrogen atoms, and the pattern repeats many times.
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Check whether the structure is made from molecules: the atoms are joined by covalent bonds, so it is a covalent molecular structure rather than an ionic lattice.
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Check the size and pattern: the chain is very long and the same arrangement of atoms repeats along the chain.
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Conclude using the definition: because it is a very large molecule made from repeating units, the structure represents a polymer.
Polymer molecules compared with giant covalent structures
This is a subtle but important distinction.
Polymers contain very large molecules. The atoms within each molecule are joined by covalent bonds, but different polymer molecules are held near each other by intermolecular forces.
Giant covalent structures, such as diamond or graphite, are different. In a giant covalent structure, atoms are joined by covalent bonds in a continuous network, not in separate molecules.
Common Mistake
Calling polymers giant covalent
Polymers are not usually described as giant covalent structures at GCSE. They have very large molecules, with covalent bonds within each molecule and intermolecular forces between molecules.
What “relatively strong” means
The spec says the intermolecular forces between polymer molecules are relatively strong. That word “relatively” matters.
Compared with simple molecular substances, polymers have stronger intermolecular forces because their molecules are much larger. This explains why polymers are usually solids at room temperature.
Compared with covalent bonds, intermolecular forces are still much weaker. If you melt or soften a polymer, you are mainly overcoming intermolecular forces between chains, not breaking all the covalent bonds in the chains.
Key Idea
Melting or softening a polymer
When a polymer melts or softens, the polymer chains can move past each other more easily. The covalent bonds within the chains are not all broken.
The exam wording you want
For a question asking why a polymer is solid at room temperature, a strong answer usually includes:
- polymer molecules are very large;
- there are relatively strong intermolecular forces between polymer molecules;
- a lot of energy is needed to overcome these forces;
- therefore the polymer is solid at room temperature.
Try to avoid vague answers like “polymers have strong bonds”. You need to say which forces or bonds you mean.
Exam technique
In the exam
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If asked to recognise a polymer, look for a very long covalent chain with a repeating unit.
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If asked to explain why polymers are solids, focus on relatively strong intermolecular forces between large polymer molecules.
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Keep the distinction clear: covalent bonds are within polymer molecules; intermolecular forces are between polymer molecules.
Self review
Check yourself
- What two features of a diagram would suggest that the substance is a polymer?
- Why are polymers usually solids at room temperature?
- What is the difference between a covalent bond and an intermolecular force in a polymer?
Recap questions
Test yourself with 5 quick questions on this guide. Answer them all correctly to complete it.
How bonding and structure are related to the properties of substances
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