Year 9 Science · Unit 2 · Lesson 17
Foundation Worksheet
Learning Goals
True or false? Fix the false ones
Circle T or F. If a statement is false, correct it on the line.
LDPE is more flexible than HDPE because LDPE’s chains have more branches.
Correct it:
A thermoset polymer can be remelted and reshaped after it has set.
Correct it:
Vulcanised rubber is a thermoset, because sulfur cross-links hold its chains together permanently.
Correct it:
Adding a plasticiser to PVC makes it harder and more rigid, which is why it is used for stiff drainpipes.
Correct it:
All polymers are made from crude oil, and none are found in nature.
Correct it:
1. Name two thermoplastic polymers and give one use of each.
Sort it
Write each material in the correct box.
Thermoplastic
Thermoset
Not a polymer
2. Explain why a thermoset chars instead of melting when it is heated strongly. Use the word cross-link in your answer.
Scenario: the heating test
In a fume cupboard, a teacher slowly heated four small plastic samples, A to D, on a hot plate. She noted the temperature at which each one softened. She tried to reshape each sample while it was warm, then recorded whether it kept the new shape after cooling. Heated plastics can give off harmful fumes, so students watched from behind a safety screen.
| Sample | What happened on heating | Reshaped while warm and kept its new shape after cooling? |
|---|---|---|
| A | softened at 110 °C | yes |
| B | did not soften; turned brown and smoked above 250 °C | no |
| C | softened at 160 °C | yes |
| D | did not soften; surface blackened at 280 °C | no |
Source: sample class results.
3. Which two samples show thermoplastic behaviour? Give the evidence from Table 1. Explain why the results for B and D are consistent with thermosets but do not prove that classification: consider a thermoplastic that does not soften in the tested range or degrades before it flows.
4. Samples A and C are LDPE (melts at about 110 °C) and polypropylene (melts at about 165 °C). Match each to its letter. Why is this heating test alone insufficient to identify the exact polymers in B and D?
5. Suggest two things the teacher should keep the same for every sample so that the test is fair.
6. In a kitchen, the handle of a frying pan is made of Bakelite, a thermoset, and cling wrap is made of LDPE, a thermoplastic. Explain why each polymer suits its job, using the structure of its chains.