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Life cycle analysis and sustainability (SACE Stage 2 Chemistry)

Syllabus dot point

“Use life cycle analysis to evaluate the sustainability and environmental impact of materials, products and chemical processes.”

8 min read

Quick answer

A life cycle analysis (LCA) assesses the total environmental impact of a product or process across its whole life: raw material extraction, manufacture, use and disposal. By accounting for energy, resources, emissions and waste at every stage, an LCA gives a fairer comparison than judging a product by one stage alone. It reveals hidden impacts (such as extraction or disposal) and shows where recycling and design changes deliver the biggest gains.

Jump to a section
  1. What this dot point is asking
  2. Lead worked calculation
  3. The stages of a life cycle analysis
  4. Why a whole-life view matters
  5. Comparing products with LCA
  6. The role of recycling
  7. Why it matters for managing resources
  8. Exam-style questions

What this dot point is asking

SACE expects you to name the LCA stages, use them to evaluate and compare products or processes, perform simple quantitative impact comparisons, and explain the role of recycling.

Lead worked calculation

Energy payback of recycling versus primary production

Producing steel from iron ore uses about 25 MJ kg−125\ \text{MJ kg}^{-1}; recycling scrap steel uses about 9 MJ kg−19\ \text{MJ kg}^{-1}. A manufacturer makes 4.0 tonnes4.0\ \text{tonnes} (4.0×103 kg4.0 \times 10^{3}\ \text{kg}) of steel products. (a) Find the energy saved by using recycled steel. (b) Express the saving as a percentage.

Energy for each route

Primary: 25×4.0×103=1.0×105 MJ25 \times 4.0 \times 10^{3} = 1.0 \times 10^{5}\ \text{MJ}. Recycled: 9×4.0×103=3.6×104 MJ9 \times 4.0 \times 10^{3} = 3.6 \times 10^{4}\ \text{MJ}.

Energy saved

saving=1.0×105−3.6×104=6.4×104 MJ\text{saving} = 1.0 \times 10^{5} - 3.6 \times 10^{4} = 6.4 \times 10^{4}\ \text{MJ}

Percentage saved

6.4×1041.0×105×100=64%\frac{6.4 \times 10^{4}}{1.0 \times 10^{5}} \times 100 = 64\%

Recycling cuts the energy by roughly two thirds. Quantifying the saving across the extraction-and-manufacture stages is exactly the kind of evidence an LCA provides.

The stages of a life cycle analysis

The four LCA stages

A life cycle analysis (sometimes "cradle to grave") examines a product across: (1) raw material extraction (mining, harvesting, and the energy and habitat impact involved); (2) manufacture/processing (energy, water, emissions and waste of production); (3) use (energy consumed and emissions during the product's working life, plus transport and distribution); and (4) disposal/end-of-life (landfill, incineration, biodegradation or recycling). Adding the impacts of all four gives the total footprint.

Why a whole-life view matters

No single stage tells the whole story

A product that looks "green" at one stage can be costly at another. An electric car has near-zero use-phase emissions but a high manufacturing impact (battery materials); a reusable cotton bag is cheap to dispose of but energy- and water-intensive to make. Only by summing impacts across all stages can two options be compared fairly. This is why LCA, not single-stage thinking, underpins genuine sustainability decisions.

Comparing products with LCA

To compare two options, build a stage-by-stage tally for each: extraction impact, manufacturing energy and emissions, use-phase energy, and disposal route. Identify where each option wins and loses, then weigh them, often per unit of service delivered (e.g. per use, per kilometre, per year). A key insight is the break-even point: a reusable item only becomes more sustainable than a disposable one after enough uses to offset its higher manufacturing impact.

The role of recycling

Recycling reduces impact at both ends of the life cycle: it avoids fresh extraction of raw materials and usually requires far less energy than primary production (dramatically so for aluminium, around 95%95\% less). It also diverts material from disposal. Designing products for easy recycling, and using recyclable materials, lowers the whole-life footprint, directly extending the green chemistry principles of waste prevention and renewable resource use.

Judging sustainability from a single stage

The most common SACE error is declaring a product sustainable (or not) based on just one stage, often manufacture or disposal. A full LCA requires considering all four stages together; a product with low manufacturing impact may have severe extraction or disposal costs. Another error is ignoring the number of uses when comparing reusable and disposable options; impact must be assessed per use over the product's whole life, not per item.

Why it matters for managing resources

Life cycle analysis is the framework that turns sustainability from a slogan into measurable, comparable evidence. It guides decisions about materials, recycling and product design, and it ties together the energy, extraction, corrosion and green chemistry ideas of this topic into a single tool for managing resources responsibly.

Exam-style questions

Questions in the style of SACE Board exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.

SACE 2021-style5 marks
A life cycle analysis compares an aluminium drink can with a glass bottle. Producing aluminium from ore requires about 210 MJ kg−1210\ \text{MJ kg}^{-1}, while recycling aluminium requires about 10 MJ kg−110\ \text{MJ kg}^{-1}. (a) State the four main stages of a life cycle analysis. (b) Calculate the percentage of energy saved by recycling aluminium rather than producing it from ore. (c) Explain why life cycle analysis gives a fairer comparison than looking at the manufacturing stage alone.
Show worked answer →

(a) The four stages are: raw material extraction; manufacture/processing; use (and transport/distribution); and disposal/end-of-life (including recycling). (2 marks)

(b) Energy saved =210−10=200 MJ kg−1= 210 - 10 = 200\ \text{MJ kg}^{-1}. Percentage saved =200210×100=95.2%= \dfrac{200}{210} \times 100 = 95.2\%. (2 marks)

(c) Life cycle analysis accounts for impacts at every stage, not just manufacture, so it captures hidden costs such as raw-material extraction, energy in use, transport and disposal. A product cheap to make may have a high extraction or disposal impact, which only a full life cycle view reveals. (1 mark)

SACE 2019-style4 marks
Compare a single-use plastic bag with a reusable cotton bag using life cycle thinking. (a) Identify one stage where the plastic bag has the lower impact and one where it has the higher impact. (b) Explain why the number of times a reusable bag is used is critical to which option is more sustainable overall.
Show worked answer →

(a) Lower impact stage for plastic: manufacture (a plastic bag uses far less energy and water to produce than a cotton bag). Higher impact stage for plastic: disposal/end-of-life (plastic persists in the environment and is rarely recycled, whereas cotton is biodegradable). (2 marks)

(b) A cotton bag has a high manufacturing impact spread over its lifetime; only if it is reused many times does its per-use impact fall below that of a plastic bag. If used only a few times, its total impact per use is higher. The break-even number of uses determines which option is genuinely more sustainable. (2 marks)

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Sources & how we know this

Based on the official SACE Board subject outline. Not yet individually reviewed by a person; always check SACE Board and your school's materials for current rules.

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