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AQA GCSE Chemistry · 8462
AQA 8462 · 4.2.2.5 Check the specification (PDF) (opens in a new tab)
A is a substance made from very large molecules. Each polymer molecule is usually a long chain containing a pattern of atoms that repeats many times. This repeated pattern is called the .
The atoms along one polymer molecule are linked by strong covalent bonds. A covalent bond forms when atoms share a pair of electrons. These bonds hold the atoms together within the chain.
A piece of polymer is not normally one continuous covalent network. It contains many separate polymer molecules lying alongside one another. Attractions called intermolecular forces act between these molecules. It is important to keep the two levels of bonding separate:
Small molecular substances have short molecules and usually only weak intermolecular forces, so their molecules can often separate or move past one another with relatively little energy. Polymer molecules are much larger and longer. The intermolecular forces between polymer molecules are relatively strong, with the long chains able to attract one another at many places.
At room temperature, there is not enough thermal energy to overcome these forces sufficiently for the polymer molecules to move around as they would in a liquid. The molecules therefore remain close together in fixed overall positions, so polymers are solids at room temperature.
This explanation depends on the forces between polymer molecules. The strong covalent bonds within each chain explain why the atoms remain joined as a large molecule, but these covalent bonds are not normally broken when a polymer melts. Melting overcomes intermolecular forces and allows the polymer molecules to move past one another.
A displayed polymer structure often shows one repeating unit inside square brackets. Bonds pass through the brackets at both ends to show that the chain continues, and the letter appears outside the brackets. The means that the unit is repeated a large number of times; it is not the symbol for an element.
Polymer diagrams show a repeating covalently bonded unit. In a polymer sample, relatively strong intermolecular forces act between separate long-chain molecules.
You may instead be given a longer section of the chain with several repeated patterns drawn out. To recognise it as a polymer, trace the covalent backbone and find a small group of atoms that occurs again and again. A line between two atoms represents a covalent bond.
Do not rely only on the diagram being large. Small molecules appear as separate, small groups of atoms. A polymer consists of separate but very large chain molecules with a repeated pattern. A giant covalent structure, by contrast, is a continuous network of covalent bonds rather than a collection of distinct polymer chains.
The relatively strong forces between the long chains require substantial energy to overcome. At room temperature, the chains cannot move freely past one another, so polymers are solids.
Look for:
Keep the distinction clear: a polymer contains separate, very large molecules, whereas a giant covalent structure is a continuous bonded network.
For a property question, name the particles or structure, identify the relevant force, and connect its strength to the energy needed to overcome it.
Do not say that polymers melt by breaking covalent bonds. On melting, the polymer molecules separate as intermolecular forces are overcome; the covalent bonds within each chain remain intact.
When identifying a polymer diagram, look for a repeating section of a long covalently bonded chain. It may be shown inside brackets with bonds crossing the brackets and an outside.
Do not describe a polymer as a giant covalent structure. A polymer sample contains separate, very large molecules, so intermolecular forces act between its chains.
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Last updated
Polymer
A substance consisting of very large, long-chain molecules made from many repeating sections joined by covalent bonds.
Covalent bond
A strong bond formed when atoms share a pair of electrons.
Intermolecular force
An attraction acting between separate molecules; it is weaker than the covalent bonds within the molecules.
Repeating unit
A group of atoms whose pattern occurs many times along a polymer molecule.
Put your knowledge into practice — try past paper questions for Chemistry
Polymer
A substance consisting of very large, long-chain molecules made from many repeating sections joined by covalent bonds.
Covalent bond
A strong bond formed when atoms share a pair of electrons.
Intermolecular force
An attraction acting between separate molecules; it is weaker than the covalent bonds within the molecules.
Repeating unit
A group of atoms whose pattern occurs many times along a polymer molecule.