Year 9 Science · Unit 2 · Lesson 18
Challenge Worksheet
Learning Goals
1. Explain why “made from plants” and “biodegradable” do not mean the same thing. Use PLA as your example.
2. The sign contains four scientific errors. Quote or describe each one, and correct it.
Find the mistakes: a café’s sign
“Our new PLA cups are made from corn, so they biodegrade anywhere, even in the ocean. Because they come from plants, making them has no environmental cost at all. Anyway, ordinary polyethylene cups would also disappear in time, because sunlight breaks them down until nothing is left. And paper cups are always the greenest choice, because paper rots in a few weeks, even in landfill.”
| What the sign says | Why it is wrong, and the correct science |
|---|---|
Data: does the cup meet the standard?
In Australia, a plastic can be certified “commercially compostable” under standard AS 4736 if at least 90% of it biodegrades within 180 days in a composting test at about 58 °C. The home-compostable standard, AS 5810, needs 90% within 12 months at ordinary outdoor temperatures. A laboratory tested pieces of a PLA cup at two temperatures, with plain paper (cellulose) as a comparison.
| Day | Paper at 58 °C | PLA at 58 °C | PLA at 25 °C |
|---|---|---|---|
| 30 | 55 | 20 | 0 |
| 60 | 80 | 55 | 1 |
| 90 | 88 | 78 | 1 |
| 120 | 92 | 88 | 2 |
| 180 | 95 | 93 | 3 |
Standards: AS 4736 and AS 5810. Results: sample laboratory results, not a real product test.
3. Does the PLA cup meet the AS 4736 requirement at 58 °C? Between which two days did it first pass 50%? Quote values from Table 1.
4. Explain why the laboratory tested plain paper at the same time as the PLA.
5. The cup’s maker prints “Compostable: better for the planet” on it. Use the 25 °C results and AS 5810 to evaluate this label for a family with a backyard compost bin. What should the label say instead?
6. Scientists make a polymer with the same useful properties as polyethylene (flexible, waterproof and cheap), but made from sugarcane instead of crude oil. It breaks down completely in industrial composting within 18 months. A supermarket chain plans to switch all its bags to the new material within five years. Evaluate whether this new polymer would solve all of polyethylene’s environmental problems, or only some of them. Consider raw materials, making the bags, using them, and what happens at the end of their life.