Lesson focus: Is changing a material always an environmental improvement?
Compare materials using evidence across their life cycleAll the stages of a product's life, from getting raw materials to making it, using it and throwing it away..
Build on earlier learning
Evaluating dataSupport a judgement with data and identify a specific limitation.
You will be successful when you can…
- Name life-cycle stages.
- Calculate cumulative energy totals.
- Find a break-even point.
- State a specific model limitation.
Words that carry the science
- life cycle
- Stages from raw material extraction to manufacture, use and end of life.
- cumulative
- A total that increases as further amounts are added.
- functional unit
- A defined service used to make alternatives comparable.
- biodegradable
- Able to be broken down by microorganisms under specified conditions.
- trade-off
- A benefit in one area accompanied by a cost in another.
Compare equal services across a whole life cycle
01Compare the same service
One bottle versus one bottle can be unfair if one is reused. Define a functional unitA fair amount to compare products by, such as "carrying 1,000 bags of shopping" rather than "one bag". such as supplying one litre of drink on 20 occasions.
02Totals change with use
In the simplified model, reusable energy is 100 + 2n and disposable energy is 10n, where n is the number of uses. Initial manufacture and repeated-use costs must both be counted.
03Models have boundaries
Transport, washing method, breakage, recycled content and end-of-life routes can alter the result. Bio-based does not necessarily mean biodegradableAble to be broken down naturally by tiny living things, such as bacteria., and biodegradation depends on conditions.
Find the first lower whole-use total
Reusable = 100 + 2n; disposable = 10n.
- Set reusable lower: 100 + 2n < 10n.
- Subtract 2n: 100 < 8n.
- Divide by 8: n > 12.5.
- The first whole number satisfying this is 13 uses; totals are 126 and 130 units.
How to make the reasoning visible: The crossover occurs between 12 and 13 uses, so the first lower reusable total is at use 13.
Life-cycle comparison
All values are illustrative energy units.
| Uses | Reusable total | Disposable total | Lower in model |
|---|---|---|---|
| 1 | 102 | 10 | Disposable |
| 5 | 110 | 50 | Disposable |
| 13 | 126 | 130 | Reusable |
| 20 | 140 | 200 | Reusable |
Why is 12 uses not enough for the reusable bottle to be lower?
Name one omitted factor and its likely direction of effect.
Use the illustrative model. After how many uses is the reusable bottle the lower-energy choice?
Check your understanding
Answer from memory, then check against the suggested answer.
What is a life cycle?
What is cumulative energy?
What is a functional unit?
What does biodegradable mean?
What does cumulative mean?
A reusable bottle breaks after eight uses. What does this model suggest?
Why might a lower energy total still not settle the overall environmental choice?
Why is a reusable bottle only better if it is actually used many times?
Exam Question Practice
Exam Question Practice
Mark points shown after marking
Illustrative data: a reusable bottle needs 100 energy units to make and 2 units to wash for each use. A disposable bottle needs 10 units for each use.
Calculate the total energy for the reusable bottle after 20 uses.
(1)Calculate the total energy for disposable bottles over 20 uses.
(1)How many energy units lower is the reusable bottle after 20 uses?
(1)Evaluate the choice of the reusable bottle using this model.
(2)Total for this question: 5 marks
Command word: Evaluation combines calculation, comparison, limitation and a conditional judgement.