Year 9 Science · Unit 2 · Lesson 18
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Learning Goals
1. What does a life cycle assessment (LCA) measure? Name the four stages of a product’s life that it covers.
Because… chain
Fill in the empty boxes to explain why plastic pollution builds up.
Data: which shopping bag has the lowest impact?
The UK Environment Agency compared the global warming potential of shopping bags over their whole life, from raw materials to disposal. Table 1 shows how many times each reusable bag must be used before its impact per use is lower than a thin HDPE bag used once for shopping.
| Bag | HDPE bag used once, then thrown away | HDPE bag used once, then every one reused as a bin liner |
|---|---|---|
| Paper | 3 | 7 |
| Thick LDPE “bag for life” | 4 | 9 |
| Non-woven polypropylene | 11 | 26 |
| Cotton | 131 | 327 |
Source: UK Environment Agency (2011), Life cycle assessment of supermarket carrier bags, report SC030148.
2. Draw a bar graph of the first column of numbers in Table 1. Label both axes.
3. Mia takes her cotton bag shopping once a week, every week of the year. How many years will it take before her cotton bag beats an HDPE bag that is used once and thrown away? Give your answer to one decimal place.
Answer years
4. The paper bag breaks down much faster than any plastic bag, yet it still needs 3 uses to beat the HDPE bag. Suggest why, using the idea of a whole life cycle. Then explain why every number rises when the HDPE bag is reused as a bin liner.
5. This study measured only global warming potential. Name one other impact a full LCA could measure, and explain how it might change which bag looks best.
6. A supermarket replaces its fruit and vegetable bags with PLA bags labelled “compostable in industrial facilities”. A survey finds that most shoppers put the used bags in their general rubbish bin, which goes to landfill. The town has no industrial composting facility. Evaluate whether the switch to PLA bags is likely to reduce the town’s plastic waste. Suggest one change that would make it work better.
7. A supermarket claims: “Switching from plastic bags to paper bags is always better for the environment.” Evaluate this claim. Use Table 1 and at least one impact that Table 1 does not include.