Synthetic Fibres and Plastics – Case-based Questions with Answers

Class 8 Science — Chapter 3: Synthetic Fibres and Plastics — Case Based Questions

Class 8

Science — Chapter 3: Synthetic Fibres and Plastics

Topic-wise, NCERT-aligned case-based questions with stepwise, easy-to-understand answers — ideal for CBSE revision and exam practice.

CBSE Board Examinations

Practice set — Chapter-wise | Case Based Questions | Aligned to NCERT

CBSE Board Examinations — Systematic order (use in planning)

Unit tests / Periodic assessments: Short/MCQ/short written tests to track progress chapter-wise.
Formative assessments: Activities, practicals, assignments and classwork to develop application skills.
Half-yearly / Mid-term: Covers the first half of the syllabus; includes short & long answer questions and sometimes case-based questions.
Annual / Summative assessment: Comprehensive paper assessing the whole syllabus; includes case-based, short-answer and long-answer questions.
Practical / Project work: Activity-based evaluation and experiments as per NCERT guidelines.

Tip: Use these case-based questions for both formative practice and summative exam preparation. Each case focuses on a core topic from the chapter: synthetic fibres, properties, plastics, polymerisation, types of plastics, uses & environmental concerns.

Case 1 — Choosing Clothing for a Sports Day

Priya needs a fabric for her school sports day uniform. She wants something that dries quickly, is light-weight and resists wrinkles. Her friend suggests a synthetic fibre.

Which synthetic fibre is most suitable for Priya's uniform — nylon, cotton, or wool? Give two reasons.

Answer:
Nylon is most suitable because: (1) It dries quickly and has low moisture absorbency, so it stays light during sports; (2) It is strong, elastic and resists wrinkles — ideal for active wear.
Note: Polyester is another good choice for similar reasons (light, quick-drying, wrinkle-resistant).

Case 2 — Label Reading

You see a garment label that says "60% Polyester, 40% Cotton". The manufacturer blended fibres to achieve desirable properties.

Explain why mixing polyester with cotton is common. List two advantages of such a blend.

Answer:
Blending polyester with cotton combines advantages of both fibres: (1) Cotton provides comfort and breathability; (2) Polyester adds strength, wrinkle resistance and quick-drying properties — overall the blend is durable, easy to maintain and comfortable.

Case 3 — Stain on a Shirt

A white shirt made of synthetic fibre develops a grease stain after lunch. The student wonders which cleaning method to use.

Are synthetic fibres easier or harder to remove grease stains from compared to cotton? Suggest a cleaning approach.

Answer:
Grease stains can cling to synthetic fibres because many are less absorbent than cotton. Use a detergent containing surfactants (a grease-fighting laundry detergent) and warm water to emulsify and remove the stain. Pre-treating with a small amount of dishwashing liquid on the stain before washing helps.

Case 4 — Choosing Rope for a Boat

A fisherman wants a rope that is strong, light, and resists rotting in water. Traditional cotton rope rots over time.

Which man-made fibre would you recommend and why?

Answer:
Recommend polyester or nylon. Both are strong and light; polyester resists UV and water damage well and keeps shape; nylon is very strong and elastic. Both resist rotting, unlike natural fibres.

Case 5 — Plastic Packaging

A snack food company uses thin, flexible plastic film to pack chips. The film is easily moulded and can be melted and reshaped with heat.

Is this film more likely to be a thermoplastic or a thermosetting plastic? Explain your answer.

Answer:
This film is a thermoplastic. Thermoplastics soften on heating and can be re-melted and reshaped. Thermosetting plastics do not soften on reheating because they form irreversible cross-linked structures.

Case 6 — Broken Handle Repair

A plastic mug handle breaks. The student considers repairing it by heating and reshaping the broken plastic.

Which type of plastic would allow such repair by reheating — polythene or bakelite? Why?

Answer:
Polythene (a thermoplastic) can be reheated and reshaped, so repair by heating may be possible. Bakelite is a thermosetting plastic and cannot be reshaped or repaired by reheating because of its cross-linked structure.

Case 7 — Environment Awareness Poster

Students prepare a poster showing the threats of plastic waste to the environment and ways to reduce it.

Name three actions the poster can recommend to reduce plastic pollution at school level.

Answer:
Actions: (1) Reduce single-use plastics — use steel or glass water bottles and lunch boxes; (2) Reuse plastic bags and containers where possible; (3) Recycle plastics properly and organise school collection drives. Also encourage alternatives like cloth bags and promote compostable materials for organic waste.

Case 8 — Identifying Fibres under Burn Test

A teacher demonstrates small burn tests (carefully and safely) to show differences between fibres: wool, cotton, and a synthetic fibre.

What characteristic flame or smell would indicate a synthetic polymer fibre (like nylon) compared to natural fibres?

Answer:
Synthetic polymer fibres often melt and form hard beads, and may give a chemical or sweetish smell on burning. Natural fibres: cotton burns with a steady flame and smells like burning paper; wool chars and smells like burning hair (nitrogen-containing protein).
Caution: Burn tests should only be done by teachers with safety precautions.

Case 9 — Rayon vs Cotton

A student reads that rayon is sometimes called 'artificial silk' and wonders why it feels like cotton but is man-made.

Explain what rayon is and how it differs from fully synthetic fibres like nylon.

Answer:
Rayon (viscose) is a regenerated cellulose fibre — made from natural cellulose (wood/pulp) processed chemically. It behaves like natural fibres (soft, breathable). Fully synthetic fibres like nylon are made from petrochemicals through polymerisation and are true synthetic polymers with different properties (more elastic, less absorbent).

Case 10 — Plastic Choice for Electrical Switch

An electrical switch casing needs a plastic that does not conduct electricity and can withstand heat from occasional current flow.

Which property of plastics makes them suitable for electrical insulation? Suggest a specific plastic used in electrical fittings.

Answer:
Plastics are generally good electrical insulators because they do not have free electrons to conduct electricity. Bakelite (a thermosetting plastic) and PVC are commonly used in electrical fittings — bakelite resists heat well and is a good insulator.

Case 11 — Recycling Symbol Confusion

Ravi notices numbered recycling symbols on plastic bottles (1–7) and asks why manufacturers use these numbers.

Briefly explain why plastics are labelled with recycling numbers and how this helps in disposal.

Answer:
Recycling numbers identify the type of polymer used (e.g., PET, HDPE, PVC, etc.). They help recyclers separate plastics by type, because different plastics need different recycling processes. Proper sorting improves recycling efficiency and reduces contamination.

Case 12 — Plastic Bottle Floating

A plastic bottle floats in water while a small clay pot sinks. Students discuss density and material types.

Explain why many plastics float in water while materials like clay sink.

Answer:
Many plastics (like polythene, polypropylene) have densities lower than water (density < 1 g/cm³), so they float. Clay and ceramics have higher densities and absorb water, causing them to sink. The structure and trapped air in plastic objects also help them float.

Case 13 — Heat Resistance in Kitchenware

You must choose a plastic container safe for hot food. Some plastics soften on heating while others do not.

Which type — thermoplastic or thermosetting — is safer for repeatedly holding hot food without deforming? Give a reason.

Answer:
Thermosetting plastics (e.g., some hard bakelites) do not melt on heating because of permanent cross-linking, so they retain shape under heat better than many thermoplastics. However, many modern food-safe plastics are specific heat-resistant thermoplastics (engineered polymers) labelled as microwave/food-safe. Always check manufacturer labels for food safety.

Case 14 — Disposal of Polythene Bags

A city struggles with polythene bag litter. Citizens propose banning thin polythene bags and using alternatives.

Give two environmental reasons to reduce single-use polythene bags and one alternative material to recommend.

Answer:
Reasons: (1) Polythene bags are non-biodegradable and remain in the environment for many years, harming wildlife; (2) They block drainage systems and increase land/river pollution.
Alternative: Reusable cloth or jute bags are biodegradable and durable alternatives.

Case 15 — Choosing Material for Raincoat

A student wants a raincoat that keeps them dry and is lightweight. They look at nylon, wool, and cotton options.

Which material should they choose and why?

Answer:
Choose nylon (or a waterproof coated polyester). Nylon is water-resistant, lightweight and dries quickly — ideal for raincoats. Cotton and wool absorb water and become heavy when wet.

Case 16 — Biodegradable vs Non-biodegradable

A class sorts waste into biodegradable and non-biodegradable piles for a school cleanliness drive.

Place these items into biodegradable or non-biodegradable: banana peel, polythene bag, paper, plastic bottle.

Answer:
Biodegradable: banana peel, paper (they decompose naturally). Non-biodegradable: polythene bag, plastic bottle (they persist for many years and require recycling or safe disposal).

Case 17 — Polymer Formation

A student learns that plastics are polymers formed by linking many small molecules (monomers).

Using simple language, explain what a monomer and a polymer are, and give one example of a polymer and its monomer.

Answer:
A monomer is a small molecule that can join with many similar molecules. A polymer is a long chain made by linking monomers. Example: Polythene is a polymer made from the monomer ethene (ethylene). Many ethene units join to form polythene.

Case 18 — Microplastics Concern

Students read that tiny plastic particles (microplastics) are found in oceans and sometimes even in food.

Suggest one school-level activity to raise awareness about microplastics and one simple way families can reduce microplastics at home.

Answer:
Activity: Organise a beach/pond clean-up and microscope session to show microplastic particles in collected samples.
At home: Reduce use of single-use plastics, avoid personal care products with plastic microbeads (use natural scrubs), and use a lint filter in washing machines to catch microfibres.

Case 19 — Safety of Plastic Toys

Parents ask whether plastic toys are safe for toddlers who might put them in their mouth.

List two safety considerations when choosing plastic toys for young children.

Answer:
Considerations: (1) Ensure the plastic is non-toxic and labelled "BPA-free" or certified safe for children; (2) Check for small detachable parts that can be choking hazards — choose sturdy, well-made toys appropriate for the child's age.

Case 20 — Long-term Effects of Plastic in Soil

A farmer notices polythene pieces in his field and worries about soil health.

Explain one harmful effect of polythene in soil and propose one practical step farmers or villagers can take to reduce such pollution.

Answer:
Harmful effect: Polythene in soil can reduce aeration and water infiltration, harming root growth and soil organisms.
Practical step: Collect and remove polythene waste for recycling or proper disposal; switch to biodegradable mulches or bury compostable materials instead of plastic, and organise community collection drives.

Synthetic Fibres and Plastics – Case-based Questions with Answers

Science — Chapter 3: Synthetic Fibres and Plastics

CBSE Board Examinations — Systematic order (use in planning)

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