Revision notes for AQA GCSE Chemistry Life cycle assessment. Open the guide for explanations and worked examples. Written against the AQA GCSE Chemistry (8462) specification, so the content matches what's examinable rather than general Chemistry background.
Life cycle assessment
What you'll learn
- What a life cycle assessment (LCA) is.
- The four main stages in a product’s life cycle.
- Why some LCA data is numerical, but some involves judgement.
- How to compare paper and plastic shopping bags fairly.
Why life cycle assessment matters
When you buy or use a product, its environmental impact did not begin at the shop — and it does not end when you throw it away.
A product may use raw materials, water and energy before it reaches you. It may also produce waste or pollution after you have finished using it.
Definition
Life cycle assessment
A life cycle assessment (LCA) is a way of assessing the environmental impact of a product over all the stages of its life.
An environmental impact is an effect on the surroundings, such as using up resources, releasing pollutants, producing waste, or using energy from fuels.
The four stages of an LCA
For GCSE Chemistry, you need to know that an LCA considers these stages:
- Extracting and processing raw materials
- Manufacturing and packaging
- Use and operation during the product’s lifetime
- Disposal at the end of its useful life
Transport and distribution can happen between all of these stages, so it should be included too.
Stage 1: extracting and processing raw materials
A raw material is a starting material taken from the Earth or from living things. Examples include crude oil, metal ores, sand, wood and cotton.
This stage may involve mining, drilling, quarrying, farming or cutting down trees. It can use land, water and energy, and may damage habitats.
Stage 2: manufacturing and packaging
Manufacturing means making the product. This may involve chemical reactions, heating, shaping, mixing or assembling.
Packaging is the material used to protect, transport or advertise the product, such as cardboard boxes, plastic wrapping or glass bottles.
Stage 3: use and operation
This is the impact while the product is being used.
For some products, this stage is very important. For example, a washing machine uses electricity and water every time it runs. For a shopping bag, the key issue is often how many times it is reused.
Stage 4: disposal
Disposal means what happens when the product is no longer useful. It may be sent to landfill, incinerated, recycled, composted, or reused in another way.
Key Idea
The whole life matters
A product that looks “environmentally friendly” at one stage may have a bigger impact at another stage. LCAs compare the whole life cycle, not just one convenient part.
Example
Sorting impacts into LCA stages
A plastic drinks bottle is made, used and then thrown away. Sort these impacts into LCA stages: crude oil is drilled; the bottle is moulded in a factory; bottles are transported to shops; used bottles are recycled or incinerated.
- Crude oil being drilled belongs to extracting and processing raw materials, because crude oil is taken from the Earth and processed before making plastic.
- The bottle being moulded in a factory belongs to manufacturing and packaging, because energy is used to shape the material into the final product.
- Bottles being moved to shops belongs to transport and distribution, which should be included between stages of the life cycle.
- Recycling or incinerating used bottles belongs to disposal, because it happens after the useful life of the bottle.
What can be measured fairly easily?
Some parts of an LCA can be quantified, which means measured using numbers.
The easier things to quantify include:
- water used, usually in cubic decimetres (dm³) or litres
- raw materials used, often in grams (g) or kilograms (kg)
- energy used, often in kilojoules (kJ) or megajoules (MJ)
- waste produced, often in grams (g) or kilograms (kg)
You might be given a table or graph and asked to compare products using ratios, percentages or totals.
Example
Calculating total energy and percentage share
A product has the following energy use over its life cycle.
| Stage | Energy used |
|---|---|
| Raw materials | 1200 kJ |
| Manufacturing | 1800 kJ |
| Transport | 300 kJ |
| Disposal | 200 kJ |
- Add the energy values for all stages:
Etotal=1200 kJ+1800 kJ+300 kJ+200 kJ=3500 kJE_{\text{total}} = 1200\text{ kJ} + 1800\text{ kJ} + 300\text{ kJ} + 200\text{ kJ} = 3500\text{ kJ}Etotal=1200 kJ+1800 kJ+300 kJ+200 kJ=3500 kJ. - Calculate the percentage due to manufacturing:
18003500×100=51.4%\frac{1800}{3500} \times 100 = 51.4\%35001800×100=51.4%. - Round sensibly: manufacturing accounts for about 51% of the total energy use, so it is the largest energy contribution in this LCA.
Tip
Read the category carefully
“Lowest energy use” does not always mean “best overall”. A product might use less energy but produce more toxic waste, use more water, or be harder to dispose of.
Why LCAs are not completely objective
Objective means based only on facts and measurements, without personal opinion.
Some LCA data is quite objective. For example, you can often measure the mass of waste produced or the amount of energy used.
However, some environmental effects are much harder to give a numerical value to. For example:
- How serious is a pollutant that harms aquatic life?
- How should habitat loss be compared with carbon dioxide emissions?
- How much importance should be given to plastic litter compared with water use?
A value judgement is a decision based partly on opinions about what matters most. LCAs often require value judgements, especially when comparing different types of environmental damage.
Common Mistake
Treating LCA as a single perfect score
An LCA is not automatically a completely objective “green score”. The conclusion can depend on which impacts are measured, how they are weighted, and what assumptions are made.
Selective and abbreviated LCAs
A selective LCA or abbreviated LCA looks at only some stages or only some types of impact.
This can be useful if there is not enough data for a full LCA. But it can also be misused. For example, a company might choose only the data that makes its product look better in advertising.
Example
Spotting a biased LCA claim
A company says, “Our plastic bag is better than a paper bag because it uses less water to manufacture.”
- The claim only uses one category: water use during manufacturing. It does not compare the full life cycle.
- Other important factors are missing, such as raw material extraction, transport, number of reuses, recycling, biodegradability and litter.
- The conclusion is therefore selective. The data may be true, but it is not enough to prove that the plastic bag is better overall.
Comparative LCAs: making the comparison fair
A comparative LCA compares two or more products that do the same job.
To make the comparison fair, you should compare the same function. For shopping bags, that means comparing the impact of carrying the same amount of shopping, not just comparing one bag with one bag.
Definition
Functional unit
A functional unit is the amount of product or service being compared. For example, “carrying one weekly shop home” is a fairer functional unit than “one bag”.
For reusable items, a useful calculation is:
impact per use=total impact for one itemnumber of uses\text{impact per use} = \frac{\text{total impact for one item}}{\text{number of uses}}impact per use=number of usestotal impact for one itemPaper bags and plastic bags
You should be able to carry out simple comparative LCAs for shopping bags made from plastic and paper.
Raw materials
Plastic bags are usually made from substances obtained from crude oil, which is a finite resource.
Paper bags are made from wood, which can be renewable if trees are replanted, but growing and processing trees still uses land, water and energy.
Manufacturing and packaging
Paper bags often need more energy and water to manufacture than lightweight plastic bags.
Plastic manufacture may use less material per bag, but it relies on crude oil and can produce chemical waste.
Transport
Paper bags are usually heavier and bulkier than plastic bags, so transporting them can require more fuel.
Use
Plastic bags can be strong and water-resistant, so they may be reused. Paper bags can tear more easily, especially when wet, but some are still reused.
The number of uses can completely change the conclusion of an LCA.
Disposal
Paper is biodegradable and can often be recycled.
Many plastics do not biodegrade quickly. Plastic bags can cause litter problems and may harm wildlife if they enter the environment. Some plastics can be recycled, but collection and processing also use energy.
Example
Comparing shopping bags per use
A simple LCA gives this energy data.
| Bag | Energy to make one bag | Transport energy | Disposal energy | Number of uses |
|---|---|---|---|---|
| Plastic | 400 kJ | 30 kJ | 20 kJ | 1 |
| Paper | 1200 kJ | 120 kJ | 30 kJ | 3 |
- Calculate the total energy for the plastic bag:
400 kJ+30 kJ+20 kJ=450 kJ400\text{ kJ} + 30\text{ kJ} + 20\text{ kJ} = 450\text{ kJ}400 kJ+30 kJ+20 kJ=450 kJ.
Since it is used once, its energy per use is 450 kJ per use. - Calculate the total energy for the paper bag:
1200 kJ+120 kJ+30 kJ=1350 kJ1200\text{ kJ} + 120\text{ kJ} + 30\text{ kJ} = 1350\text{ kJ}1200 kJ+120 kJ+30 kJ=1350 kJ. - Divide by the number of uses for the paper bag:
1350 kJ3=450 kJ per use\frac{1350\text{ kJ}}{3} = 450\text{ kJ per use}31350 kJ=450 kJ per use. - Compare the result: in this simplified energy-only LCA, the bags are equal per use. A final judgement would still need other impacts, such as water use, raw materials, litter and recycling.
Common Mistake
Do not overclaim
If an LCA only compares energy use, your conclusion should only be about energy use. Do not say “therefore it is better for the environment overall” unless the data supports the whole conclusion.
How to write LCA conclusions
A strong GCSE answer usually does three things:
- compares specific stages of the life cycle
- uses data where it is available
- acknowledges limitations, such as missing pollutant effects or assumptions about reuse
For example, instead of writing:
“Paper bags are better because they are natural.”
A stronger answer is:
“Paper bags are made from a renewable raw material and are biodegradable, but they may use more water and energy during manufacture and more fuel during transport because they are heavier. The better choice depends on reuse and disposal.”
Exam technique
In the exam
- Name the life-cycle stage you are discussing: raw materials, manufacture and packaging, use, disposal, or transport.
- Compare like with like: use the same functional unit, especially for reusable products such as shopping bags.
- Use numbers when given, including totals, percentages or impact per use.
- Be cautious with conclusions: mention missing data, value judgements, or selective LCAs if the evidence is incomplete.
Self review
Check yourself
- What are the four main stages included in a life cycle assessment?
- Why is it harder to put a number on pollutant effects than on energy use?
- A paper bag takes more energy to make than a plastic bag but is reused several times. How would you compare them fairly?
Life cycle assessment and recycling
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