Revision notes for AQA GCSE Chemistry Ceramics, polymers and composites. 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.
Ceramics, polymers and composites
What you'll learn
- How common glasses and clay ceramics are made.
- Why different forms of poly(ethene) can come from the same monomer.
- How thermosoftening and thermosetting polymers differ in structure.
- How composites combine materials, and how to choose materials using property data.
The big idea: structure affects properties
A material is a substance chosen to make an object because of its useful properties. In this topic, you compare four important groups: ceramics, polymers, composites and metals.
Definition
Physical property
A physical property is a measurable feature of a material that does not involve making a new substance. Examples include density, strength, hardness, melting point, electrical conductivity and thermal conductivity.
When choosing a material, you are usually asking:
- Is it strong enough?
- Is it light enough?
- Will it survive heat, water or chemicals?
- Does it need to conduct electricity or insulate?
- Is it cheap and easy to shape?
Ceramics
Definition
Ceramic
A ceramic is a hard material made by heating substances such as clay or sand-based mixtures to high temperatures.
Ceramics are usually hard, heat resistant and electrical insulators, but many are also brittle, meaning they can crack or shatter when hit.
Glasses
Most glass we use is soda-lime glass. It is made by heating a mixture of:
- sand, which is mainly silicon dioxide, SiO₂
- sodium carbonate, Na₂CO₃
- limestone, which is calcium carbonate, CaCO₃
The mixture is heated until it melts, then it can be shaped and cooled.
Another important type is borosilicate glass, made from sand and boron trioxide, B₂O₃. Borosilicate glass melts at a higher temperature than soda-lime glass, so it is useful for objects that may get very hot, such as laboratory glassware and some cooking dishes.
Key Idea
Choosing a glass
Use soda-lime glass for many everyday items such as windows and bottles. Use borosilicate glass when higher heat resistance is needed.
Clay ceramics
Clay ceramics, including pottery and bricks, are made by:
- shaping wet clay
- heating it strongly in a furnace, which is a very hot oven
Heating removes water and causes the particles to bond together more strongly, so the object becomes hard and rigid.
Polymers
Definition
Polymer and monomer
A polymer is a large molecule made from many repeating smaller molecules. The small molecules are called monomers.
A very common polymer is poly(ethene). It is made from the monomer ethene.
n CH2=CH2(g)→[−CH2−CH2−]n(s)n\,\text{CH}_2{=}\text{CH}_2\text{(g)} \to \left[-\text{CH}_2-\text{CH}_2-\right]_n\text{(s)}nCH2=CH2(g)→[−CH2−CH2−]n(s)The properties of polymers depend on:
- the monomers used
- the conditions used to make the polymer, such as temperature, pressure and catalysts
A catalyst is a substance that speeds up a reaction without being used up.
Low density and high density poly(ethene)
Both low density poly(ethene), LDPE, and high density poly(ethene), HDPE, are made by polymerising ethene. They are different because they are made under different conditions, which affects the structure of the polymer chains.
LDPE
LDPE is made using high pressure and a controlled temperature. Its polymer chains are more branched, so they cannot pack closely together.
That gives LDPE:
- lower density
- more flexibility
- uses such as plastic bags and flexible film
HDPE
HDPE is made at lower pressure and temperature using a catalyst. Its polymer chains are mostly straighter, so they pack more closely together.
That gives HDPE:
- higher density
- greater strength and rigidity
- uses such as milk bottles, pipes and containers
Example
Explaining LDPE and HDPE from the same monomer
Two samples are both made by polymerising ethene. Sample A is made at high pressure and has branched chains. Sample B is made at lower pressure using a catalyst and has straighter chains.
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Identify the shared monomer. Both samples are made from ethene, so both are forms of poly(ethene), not completely different polymers.
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Link conditions to chain shape. High-pressure production gives more branching, so Sample A is LDPE. Catalyst-controlled lower-pressure production gives straighter chains, so Sample B is HDPE.
-
Link chain shape to density. Branched chains cannot pack closely, so Sample A has lower density and is more flexible. Straighter chains pack closer together, so Sample B has higher density and is more rigid.
Common Mistake
Same monomer does not mean same properties
LDPE and HDPE are both made from ethene, but they do not have identical properties. Manufacturing conditions affect the polymer structure, which changes the properties.
Thermosoftening and thermosetting polymers
Polymers can also be grouped by what happens when they are heated.
Definition
Thermosoftening polymer
A thermosoftening polymer softens or melts when heated, then hardens again when cooled. It can usually be remoulded.
Thermosoftening polymers have separate polymer chains. There are weak forces between the chains. When heated, these weak forces are overcome, so the chains slide past each other.
Definition
Thermosetting polymer
A thermosetting polymer does not melt when heated because its chains are joined by strong covalent cross-links.
A cross-link is a bond connecting one polymer chain to another. In thermosetting polymers, these cross-links hold the chains in a rigid network. If heated strongly enough, the polymer may char or decompose instead of melting.
Example
Predicting polymer behaviour from structure
A polymer has long chains joined together by many covalent cross-links. It is used to make an electrical plug casing.
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Use the structure. The chains are joined by cross-links, so the chains cannot move freely past each other.
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Predict the effect of heating. Heating will not simply overcome weak forces between separate chains, because the network is held together by strong covalent bonds.
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Classify the polymer. The polymer is thermosetting, so it is suitable for a plug casing because it will not soften easily if it gets warm.
Tip
Thermosoftening vs thermosetting
Think: softening = separate chains can slide; setting = cross-linked network stays set.
Composites
Definition
Composite
A composite is a material made from two or more different materials combined together, so the final material has useful properties from each part.
Most composites have two main parts:
- a matrix or binder, which surrounds and holds the material together
- a reinforcement, which is fibres or fragments that add strength, stiffness or toughness
Examples of composites you should be able to recall include:
| Composite | What it contains | Why it is useful |
|---|---|---|
| Reinforced concrete | Concrete with steel rods or mesh | Concrete resists compression; steel improves tensile strength |
| Fibreglass | Glass fibres in a polymer resin | Strong, lightweight and mouldable |
| Carbon fibre composite | Carbon fibres in a polymer matrix | Very strong and stiff for its mass |
| Plywood | Thin wood layers glued with grains in different directions | Stronger and less likely to split than one layer of wood |
Key Idea
Why composites are useful
A composite is designed so that the matrix holds everything together while the reinforcement improves properties such as strength, stiffness or toughness.
Comparing and choosing materials
In exams, you may be given data about materials and asked to choose the best one for a job.
Definition
Quantitative comparison
A quantitative comparison uses numbers with units, not just words such as “stronger” or “lighter”.
Here are common property patterns:
| Material type | Often useful because | Often limited because |
|---|---|---|
| Ceramics | Hard, heat resistant, electrical insulators | Brittle, difficult to reshape |
| Polymers | Low density, easily shaped, electrical insulators | Lower melting points than metals or ceramics |
| Composites | Properties can be designed for a specific job | Can be expensive or difficult to recycle |
| Metals | Strong, malleable, good conductors | Often higher density; some corrode |
Important properties include:
- density: mass per unit volume; lower density means lighter for the same volume
- tensile strength: how well a material resists being pulled apart
- hardness: resistance to scratching or denting
- thermal conductivity: how well heat passes through
- electrical conductivity: how well electric charge passes through
- melting point: the temperature at which a solid becomes a liquid
Example
Choosing a material using property data
A manufacturer needs a lightweight casing with volume 50 cm³. It must be an electrical insulator and must have a tensile strength of at least 200 MPa.
Given data:
- Soda-lime glass: density 2.5 g/cm³, tensile strength 40 MPa, electrical insulator
- Aluminium alloy: density 2.7 g/cm³, tensile strength 300 MPa, electrical conductor
- Glass-fibre composite: density 1.9 g/cm³, tensile strength 450 MPa, electrical insulator
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Eliminate materials that fail essential requirements. Soda-lime glass is an insulator, but its tensile strength is only 40 MPa, which is below 200 MPa. Aluminium alloy is strong enough, but it conducts electricity, so it fails the insulation requirement.
-
Calculate the mass of the remaining suitable material. For the glass-fibre composite, use m=ρVm = \rho Vm=ρV:
m=1.9 g/cm3×50 cm3=95 gm = 1.9\,\text{g/cm}^3 \times 50\,\text{cm}^3 = 95\,\text{g}m=1.9g/cm3×50cm3=95g -
Select and justify. Glass-fibre composite is the best choice because it is an electrical insulator, its tensile strength is above 200 MPa, and its mass for the required volume is only 95 g.
Common Mistake
Forgetting the job the material must do
Do not choose a material just because it has one impressive property. A material must meet all the important requirements for the use.
Exam technique
In the exam
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Identify the material family first: ceramic, polymer, composite or metal.
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Link properties to structure where possible, especially for thermosoftening and thermosetting polymers.
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When given numbers, compare them with units and use calculations such as m=ρVm = \rho Vm=ρV if density and volume are provided.
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For “choose a material” questions, state the chosen material and justify it using the required properties for the job.
Self review
Check yourself
- Why does HDPE have a higher density than LDPE even though both are made from ethene?
- How does the structure of a thermosetting polymer stop it from melting?
- What are the matrix and reinforcement in a composite, and what does each part do?
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