Plastic waste and recycling
Key idea: Physical recycling melts and reshapes suitable plastics, keeping the polymer in use. Cracking breaks polymer chains into smaller molecules that can be used as fuel.
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Guided lesson
Guided steps
- Compare reshaping plastic with breaking its molecules.
- Explain why plastic waste lasts and harms living things.
- Compare what each treatment needs and what it produces.
- Track the molecules through recycling, cracking and burning.
- Write a complete comparison of the two methods.
- Decide what else the school needs to check.
- Recommend what the school should do with each batch.
- Explain two ways discarded plastic causes pollution.
- Name the process used to make the pellets.
- Explain what melting does to the polymer molecules.
- Follow the carbon from plastic into burning fuel.
- Choose a route that meets each batch’s needs.
- Explain your choices and one environmental drawback.
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ContinueA plastic bag can be useful for minutes and remain as waste for years. In this lesson you explain why plastic waste causes problems and compare two treatment methods. You then make a choice from evidence about the waste, rather than assuming one method is always best.
Start with Addition polymer structures and uses: a polymer consists of long molecules, and melting a thermoplastic changes its shape without deliberately breaking its polymer chains.
What happens to the material?
Keep track of the polymer molecules:
- Physical recycling reshapes suitable polymer material for another use.
- Chemical treatment by cracking breaks polymer molecules into smaller molecules. The syllabus example uses the products as fuel; burning that fuel uses up the material rather than keeping it as plastic.
Why plastic waste persists
Poly(ethene) is useful because it is unreactive, but that also makes it non-biodegradable under ordinary disposal conditions: microorganisms do not readily break it down. Discarded plastic therefore causes pollution problems:
- it lasts for a very long time in landfill sites, which fill up;
- litter spoils land and blocks drains, and plastic in rivers and seas harms animals that eat it or become trapped in it;
- burning plastic waste releases carbon dioxide and can release toxic gases.
Compare treatment methods
| Method | What happens | Benefit | Limitation |
|---|---|---|---|
| physical recycling | poly(ethene) waste is sorted, cleaned and cut into small pieces, which are melted and made into pellets for new products | the polymer is used again; less new crude oil is needed | needs clean, sorted waste of one polymer; collecting, cleaning and melting use energy |
| chemical recycling | plastic waste is cracked into smaller molecules that are used as fuel | can treat some suitable waste that is unsuitable for physical recycling | needs an appropriate, accepted feedstock and energy; the fuel releases carbon dioxide when burned |
Cracking is not a way to accept every mixture. Facilities have limits on the polymers, additives and contamination they can handle. Check what the actual facility accepts.
For either method, weigh three kinds of issue:
- social: people must sort and return their waste, and collection must be convenient; recycling creates jobs;
- economic: collecting, sorting, cleaning and heating cost money, which must be compared with the value of the pellets or fuel produced;
- environmental: less plastic in landfill and the sea, and less crude oil used, but energy is used and carbon dioxide is released during processing.
Check your understanding 1 Compare reshaping and breaking molecules
For the two processes in the table, choose what happens to the polymer molecules. If fuel made by cracking burns completely, identify where its carbon goes.
Show answer
Physical recycling keeps polymer material in use. Cracking changes the molecules into smaller ones; when its resulting fuel burns, the carbon enters carbon dioxide. These decisions support the complete process comparison, including benefits, limitations and a justified choice, in the next activity.
Exam-style question 1
Evaluate two ways to recycle plastic
Examination question
Compare physical recycling of clean poly(ethene) with chemical treatment of suitable mixed plastic waste by cracking. Assume a facility accepts this mixture. Describe each process, give one benefit and one limitation, and reach a justified choice. [4 marks] Work on paper or in the optional notes, then compare your complete answer with the model and four criteria.
Write a balanced comparison
Show solution step by step
Describe physical recycling
1 markMethod
State what happens to the clean poly(ethene).Reason
The polymer is kept and only reshaped.Working
Answer
Clean poly(ethene) is sorted, cut into small pieces, melted and made into pellets.
Describe chemical recycling
1 markMethod
State what happens to the mixed waste.Reason
The polymer molecules are broken into smaller ones.
Working
Answer
Plastic waste is cracked into smaller molecules that are used as fuel.
Give a benefit and a limitation
1 markMethod
Compare what each method needs and what it produces.
Reason
A comparison needs a point on each side.Working
Answer
Physical recycling keeps the polymer in use but needs clean, sorted waste. Cracking can treat this accepted mixture but breaks the polymer into fuel and uses energy. The prompt gives no figures to decide which method uses more energy.
Reach a justified choice
1 markWorking
Answer
Use physical recycling for the clean poly(ethene), because it can be remelted into a reliable product. Consider cracking for this accepted mixture if it cannot be sorted and remelted reliably. Check energy use, emissions and costs before making a broader comparison.
Self-mark with the mark scheme
Tick each mark point that your answer includes.
Credit a conditional choice linked to the type of waste, not a claim that one method is always best.
Apply the comparison to a school
Check your understanding 2 Check what the collection price tells you
A school has clean poly(ethene) bags and mixed food packaging. The company charges more for mixed collection, but no local facility’s accepted feedstock is known. Decide what the price proves and what needs checking before recommending cracking.
Show answer
The collection price is economic evidence only. Check whether the waste can first be sorted and cleaned and whether a suitable facility accepts the remainder. Now give the full social, economic and environmental comparison and a conditional recommendation.
Check your understanding 3 Advise a school
A school collects its used plastic. Half of it is clean poly(ethene) bags; the rest is mixed food packaging. The recycling company charges more to collect mixed waste. Give one social, one economic and one environmental point the school should consider, and recommend what to do with each batch. You have not been told which wastes local facilities accept.
Work on paper or in the optional notes. Compare social, economic and environmental points, both batches and the missing facility-acceptance evidence.
Show answer
Social: students must sort the bags from the packaging, which needs clear bins and instructions. Economic: sorting lowers the collection charge, while mixed waste costs more to collect and process. Environmental: recycling keeps plastic out of landfill and the sea, but collection and processing use energy. Recommendation: send the clean bags for physical recycling into pellets if the local facility accepts them. First check whether the mixed packaging can be sorted and cleaned. Consider cracking only if a suitable facility accepts the remaining material; the collection price alone cannot tell you whether it can be treated.
Check your understanding
Check your understanding 4 Explain two pollution problems
Explain two ways discarded poly(ethene) can cause pollution.
Work on paper or in the optional notes. Compare persistence or landfill space with a distinct litter, drainage or way animals are harmed.
Show answer
It persists in landfill, taking up space. Litter can block drains; animals may eat it or become trapped in it.
Check your understanding 5 Identify the process before explaining it
A factory melts clean poly(ethene) pieces into pellets. Choose the method and whether its purpose is to break the polymer molecules into smaller ones.
Show answer
This is physical recycling. Melting lets the chains move past each other; cracking breaks them into shorter molecules.
Check your understanding 6 Explain what happens to the molecules
A factory melts clean poly(ethene) pieces and makes pellets. Name the method and explain whether it deliberately breaks polymer chains.
Work on paper or in the optional notes. Compare melting/reshaping with the polymer molecules retained, rather than claiming a cracking reaction.
Show answer
Physical recycling. The material melts and is reshaped; the process does not deliberately break the polymer chains into small molecules.
Check your understanding 7 Follow the molecules into fuel and carbon dioxide
Another factory cracks suitable plastic waste into fuel. Explain the molecular change and what happens to the carbon when the fuel burns completely.
Work on paper or in the optional notes. Compare the breaking of long molecules, subsequent complete combustion and loss of the original plastic material.
Show answer
Cracking breaks long polymer molecules into smaller molecules. Complete combustion of the resulting fuel converts its carbon to carbon dioxide; the original plastic material is not kept in use.
Check your understanding 8 Use the facility requirements
A town has 30% clean poly(ethene) film and 70% contaminated mixed packaging. The local physical plant accepts only clean poly(ethene); a cracking plant 200 km away accepts the mixed packaging. Choose a route that keeps the clean film as polymer material and a route that the plants accept for the mixed packaging.
Show answer
Use the local physical route for the clean film and the accepting cracking plant for the mixed packaging. The next activity asks for the full recommendation and an environmental drawback; accepted feedstock alone cannot tell you which route has the lowest overall emissions.
Check your understanding 9 Explain the accepted routes and an environmental drawback
A town’s plastic waste is 30% clean poly(ethene) film and 70% food-contaminated mixed packaging. The local physical-recycling plant accepts only clean poly(ethene). A cracking plant 200 km away accepts the mixed packaging. Recommend a method for each part, and give one environmental drawback of the method you choose for the mixed packaging.
Work on paper or in the optional notes. Compare both stated acceptance limits with transport fuel/emissions or the carbon dioxide released when the cracked fuel burns.
Show answer
Send the clean poly(ethene) film for physical recycling, which keeps the polymer in use. Send the mixed packaging, which the local plant cannot accept, for cracking. Drawbacks for the mixed packaging: carrying it 200 km uses fuel and releases carbon dioxide, and the fuel made by cracking releases carbon dioxide when it burns, so the material is not kept as plastic.
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Syllabus and review details
- SEC G3 Combined Science Chemistry component 2027 · 2027
Chemistry content structure and subject content, PDF pages 26–36
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