Year 9 Science · Unit 2 · Lesson 17
Challenge Worksheet
Learning Goals
1. A set epoxy glue cannot be softened by heating, but a polystyrene cup can. Explain the difference in terms of the forces or bonds between chains.
Class investigation: the stretch test
A group cut a 2 cm wide strip from an LDPE bread bag and a 2 cm wide strip from an HDPE shopping bag. They hung masses from each strip and measured how much it stretched.
| Mass hung (g) | LDPE strip | HDPE strip |
|---|---|---|
| 100 | 3 | 1 |
| 200 | 7 | 2 |
| 300 | 12 | 3 |
| 400 | 18 | 4 |
| 500 | broke | 6 |
Source: sample class results.
The group wrote this conclusion: “HDPE stretched less, so HDPE must have longer chains. This proves that chain length controls stiffness. It also shows that HDPE is a thermoset.”
2. At 300 g, how many times further did the LDPE strip stretch than the HDPE strip? Up to 400 g, which strip’s stretch rose by the same amount for each extra 100 g?
3. Identify three errors in the group’s conclusion. For each, explain what the correct science is.
Plan a better investigation
The strips came from bags of different thickness. Plan a fair test of the question: “Does branching in polyethylene affect how far a strip stretches under a load?”
Hypothesis
If … then … because …
Variables
Method
One limitation
4. A manufacturer claims: “We can recycle thermoset polymers just as easily as thermoplastics. We simply melt them down and mould them into new products.” Explain the scientific error in this claim, and describe two things that can be done with thermosets at the end of their life instead.
5. A car’s dashboard must stay rigid at up to 80 °C in a parked car, stay stable in shape for 15 years, and, the car maker hopes, be recycled when the car is scrapped. Polypropylene, a thermoplastic, melts at about 165 °C. Evaluate whether a thermoplastic such as polypropylene or a thermoset would be the better choice.