GCSE · Chemistry · AQA · Spec 8462

Life cycle assessment

Paper bag or plastic bag? Which is greener depends on the evidence — and on who is weighing it.

Practice data · made up for this lesson

Paper or plastic? Weigh it up yourself

The claim

On this practice data, the paper bag is the more sustainable option.

Place each piece of evidence to load the balance. Mark the strong ones — they count double.

Raw materials stage: obtaining and processing the raw materials uses 1.0 MJ of energy for the paper bag, but only 0.5 MJ for the plastic bag.
Evidence 1: does it support or challenge the claim?SupportsChallenges
Strong evidence
Manufacturing stage: making the plastic bag releases 5 g of an air pollutant; making the paper bag releases 2 g.
Evidence 2: does it support or challenge the claim?SupportsChallenges
Strong evidence
Manufacturing stage: making the paper bag uses 3.0 litres of water; making the plastic bag uses 0.2 litres.
Evidence 3: does it support or challenge the claim?SupportsChallenges
Strong evidence
Disposal stage: the paper bag leaves 1 g of waste in landfill; the plastic bag leaves 6 g.
Evidence 4: does it support or challenge the claim?SupportsChallenges
Strong evidence
Manufacturing stage: making the paper bag releases 4 g of a water pollutant; making the plastic bag releases 1 g.
Evidence 5: does it support or challenge the claim?SupportsChallenges
Strong evidence
Transport, added up across every stage: 0.2 MJ of energy for the paper bag, 0.4 MJ for the plastic bag.
Evidence 6: does it support or challenge the claim?SupportsChallenges
Strong evidence

One bag, start to finish

Open each stage to see what the assessment asks there.

Check your thinking

So is an LCA just a measurement?

Your friend has just read an LCA of the two bags and says: 'It's science — everything in it is measured, so whatever it says is the answer. End of discussion.'

Which of these is closest to what you think right now?

My friend is right: every impact in an LCA is measured exactly, so its verdict is pure fact.The tricky part is measuring the water and energy used. Scoring pollution is simple.Some parts are easy to measure, like water and energy use, but scoring the effects of pollutants takes value judgements — so it is not purely objective.If there is any judgement in it at all, an LCA is just opinion. It tells us nothing.

How sure are you?

I'm sureI think soI'm guessing

My friend is right: every impact in an LCA is measured exactly, so its verdict is pure fact.

This is a common misconception

Part of that is true, and it is worth knowing which part. The use of water, resources and energy sources, and the production of some wastes, can be fairly easily quantified. But putting a number on the effect of a pollutant is less straightforward: it needs value judgements. So an LCA is not a purely objective process, and its verdict is not pure fact.

The tricky part is measuring the water and energy used. Scoring pollution is simple.

This is a common misconception

It is the other way round. Water, resources, energy sources and some wastes are the parts that can be fairly easily quantified. Allocating numerical values to pollutant effects is the part that is less straightforward — that is where the value judgements come in.

Some parts are easy to measure, like water and energy use, but scoring the effects of pollutants takes value judgements — so it is not purely objective.

This matches the evidence

Exactly right. Water, resources, energy sources and some wastes can be fairly easily quantified. Allocating numbers to pollutant effects needs value judgements, which is why an LCA is not a purely objective process — and why it matters who made those judgements.

If there is any judgement in it at all, an LCA is just opinion. It tells us nothing.

This is a common misconception

That throws out far too much. Plenty of an LCA can be fairly easily quantified — water, resources, energy sources and some wastes — and it can reveal the costs to people or the environment at each stage of a product's life. The judgement comes in when pollutant effects are given numbers. So use the data, and look carefully at the judgements.

Measured amounts plus judged pollutant effects: an LCA is useful, but it is not purely objective.

Read it like a detective

Where does this leaflet's reasoning break down?

A company that makes plastic bags hands out this leaflet. Pick the line where its reasoning goes wrong.

The company's leaflet — which line goes wrong?

We carried out a life cycle assessment of our plastic bag and of a paper bag.
To keep the report short, we assessed only the manufacturing stage.
At that stage, our plastic bag used less energy than the paper bag.
So our plastic bag is the more sustainable option — the LCA proves it.

Your turn to write it

Paper or plastic — on paper

Practice data (made up for this lesson), per bag: Raw materials stage — energy used: paper bag 1.0 MJ, plastic bag 0.5 MJ Manufacturing stage — water used: paper bag 3.0 litres, plastic bag 0.2 litres Manufacturing stage — air pollutant released: paper bag 2 g, plastic bag 5 g Manufacturing stage — water pollutant released: paper bag 4 g, plastic bag 1 g Use stage — no difference: both bags are used for the same number of trips Disposal stage — waste left in landfill: paper bag 1 g, plastic bag 6 g Transport, all stages added up — energy used: paper bag 0.2 MJ, plastic bag 0.4 MJ

Use the practice data to carry out a simple comparative LCA of the two bags, and evaluate which bag is the more sustainable option. [6 marks]

Your answer

0 words · your answer stays on this page and is not sent anywhere.

The mark scheme

Compares the energy figures: the plastic bag uses less energy for its raw materials (0.5 MJ against 1.0 MJ) but more for transport (0.4 MJ against 0.2 MJ). (1 mark)
Compares water use: the plastic bag uses far less water in manufacturing (0.2 litres against 3.0 litres). (1 mark)
Compares the pollutants: the paper bag releases less air pollutant (2 g against 5 g) but more water pollutant (4 g against 1 g). (1 mark)
Compares disposal: the paper bag leaves less waste in landfill (1 g against 6 g). (1 mark)
Reaches a clear conclusion on which bag is the more sustainable option, justified by the comparisons made — either bag can earn this. (1 mark)
Gives a limitation, such as: deciding how much each pollutant's effect counts needs a value judgement, so the LCA is not purely objective. (1 mark)

Model answer

At the raw materials stage the plastic bag uses half the energy of the paper bag (0.5 MJ compared with 1.0 MJ), and in manufacturing it uses far less water (0.2 litres compared with 3.0 litres). The paper bag does better elsewhere: it releases less air pollutant (2 g compared with 5 g), leaves less waste in landfill (1 g compared with 6 g) and uses less energy on transport (0.2 MJ compared with 0.4 MJ), although it releases more water pollutant (4 g compared with 1 g). The use stage shows no difference. Overall, I judge the plastic bag to be the more sustainable option, because its savings in energy and especially water are large, and water and energy use can be fairly easily quantified. However, this rests on a value judgement: deciding how much the air and water pollutants' effects count is not straightforward, so someone who weights air pollution and landfill waste more heavily could fairly conclude that the paper bag is better.

A strong answer compares both bags stage by stage with the numbers, commits to a verdict, and names the judgement that verdict rests on.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Follow a product from its raw materials to the end of its life — and find out why two people can read the same assessment differently.

What you need to know

An LCA assesses a product's environmental impact at each stage of its life: raw materials, manufacturing and packaging, use and operation, and disposal — with transport and distribution included at each stage.
At the end of its useful life a product can be repaired, recycled, incinerated or thrown away.
Water, resources, energy sources and some wastes can be fairly easily quantified; allocating numbers to pollutant effects needs value judgements.
To compare two shopping bags, use the information given to complete an LCA for each, then evaluate the data to decide which is the more sustainable option.

The big picture

A life cycle assessment (LCA) follows a product through its whole life and assesses its environmental impact at each stage: obtaining and processing raw materials, manufacturing and packaging, use, and disposal — with transport and distribution counted at every stage. Along the way it can reveal the costs to people or the environment. The use of water, resources and energy sources, and the production of some wastes, can be fairly easily measured, but putting numbers on the effects of pollutants needs value judgements, so an LCA is not purely objective. That is why you always ask who reported an LCA, and whether it left anything out.

Key points

1An LCA follows a product from its raw materials to the end of its life — not just the time it spends in the factory.

2Transport and distribution are counted at every stage, not saved up as a stage of their own.

3At each stage, an LCA can reveal the costs to individuals or the environment.

4Scoring pollutant effects needs value judgements, so an LCA is not a purely objective process.

5Selective or abbreviated LCAs can be misused to reach a conclusion decided in advance — and because LCAs are often subjective, consider the vested interests of whoever reports the findings.

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Worked example

Problem

Using this lesson's practice data (made up for practice), which bag uses less energy across the figures given for energy — and does that settle which bag is the more sustainable option?

⚠ Watch out

Treating an LCA's verdict as pure fact. The use of water, resources and energy, and some wastes, can be measured fairly easily — but deciding how much a pollutant's effect counts is a value judgement.

🧠

Memory hook

Get it, make it, use it, lose it — and move it at every step. Then ask the big question: who is doing the weighing?

✓

Check yourself

Two people read the same LCA data for a paper bag and a plastic bag and pick different winners. How can both of them have read every number correctly?

Flashcards

(11)

What does a life cycle assessment (LCA) assess?

The environmental impact of a product, stage by stage, across its whole life.

What are the four stages of a product's life in an LCA?

Obtaining and processing raw materials; manufacturing and packaging; use and operation during its lifetime; disposal at the end of its useful life.

Is transport a separate stage in an LCA?

No. Transport and distribution are included at each stage, from getting the raw materials to disposal.

What has to happen before a product can be made?

Its raw materials need to be obtained and processed.

What four things can happen to a product at the end of its useful life?

It can be repaired, recycled, incinerated or thrown away.

What can an LCA reveal at each stage of a product's life?

The costs to individuals or the environment.

Which things in an LCA can be fairly easily quantified?

The use of water, resources and energy sources, and the production of some wastes.

Why is an LCA not a purely objective process?

Allocating numerical values to pollutant effects is less straightforward and requires value judgements.

Why should you consider who reported an LCA's findings?

LCAs are often subjective, so the people reporting on them may have vested interests.

What is the danger of a selective or abbreviated LCA?

It can be misused to reach a pre-determined conclusion — for example, to support a claim in an advert.

How do you carry out a simple comparative LCA of two shopping bags?

Use the information given to complete an LCA for each bag, then evaluate the data to decide which is the more sustainable option.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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Paper or plastic? Weigh it up yourself

So is an LCA just a measurement?

Where does this leaflet's reasoning break down?

Paper or plastic — on paper

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