Chapter Overview and Weightage
Synthetic Fibres and Plastics is a concept-heavy chapter in Class 8 Science that forms the foundation for understanding polymers in higher classes. In CBSE board-pattern exams and school-level tests, this chapter typically carries 5–8 marks.
Questions are predominantly definition-based, comparison-based (nylon vs rayon, thermoplastic vs thermosetting), and application-based (why synthetic fibres melt near fire).
This chapter overlaps with Class 10 Carbon and its Compounds (synthetic polymers) and Class 12 Polymers. Understanding the basic polymer concept here makes those chapters much easier later.
| Question Type | Expected Marks |
|---|---|
| Define terms (fibre, polymer, monomer) | 1 mark each |
| Differences (thermoplastic vs thermosetting) | 3–4 marks |
| Advantages/disadvantages of synthetic fibres | 2–3 marks |
| Application questions (why not wear synthetic near fire) | 2 marks |
| Matching (fibre names to properties) | 2 marks |
Key Concepts You Must Know
What is a synthetic fibre?
A synthetic fibre is a man-made material formed by joining many small chemical units (monomers) into long chains (polymers). Unlike natural fibres (cotton, wool, silk — from plants and animals), synthetic fibres are entirely manufactured from chemicals, usually derived from petroleum.
Polymer and Monomer:
A monomer is the small repeating unit; a polymer is the long chain formed by joining many monomers. Nylon is a polymer of hexamethylene diamine and adipic acid monomers.
Four Major Synthetic Fibres:
-
Rayon (Artificial Silk):
- Made from cellulose (chemically treated wood pulp)
- Technically semi-synthetic (natural raw material, chemical processing)
- Soft, silky appearance — used in dresses, saris
- Cheaper than real silk
-
Nylon:
- First fully synthetic fibre
- Strong, elastic, lightweight, resistant to chemicals
- Used in toothbrush bristles, ropes, parachutes, socks, stockings
- Can withstand heavy loads without breaking
-
Polyester (Terylene):
- Made from ester monomers
- Wrinkle-resistant, does not absorb moisture, dries quickly
- Used in clothes (often blended with cotton — Poly-cotton blends)
- PET (polyethylene terephthalate) is a polyester used in bottles and films
-
Acrylic:
- Resembles wool in appearance and warmth
- Much cheaper than natural wool
- Used in sweaters, blankets, shawls
- Advantage over wool: not eaten by moths, does not shrink
Important Concepts — Plastics
What is plastic?
Plastic is a polymer that can be moulded into different shapes. Like synthetic fibres, plastics are made from petrochemical monomers.
Two types of plastics — this is high-priority for exams:
Thermoplastics:
- Soften on heating, can be remoulded and reshaped
- Examples: PVC, polyethylene (polythene)
- Uses: toys, combs, buckets, carry bags
Thermosetting plastics:
- Once set (hardened), cannot be remoulded by heating — they char or break
- Examples: Bakelite, Melamine
- Uses: electrical switches (Bakelite), crockery and floor tiles (Melamine)
Thermoplastic: Heating → Soft → Reshapeable → Cool → Hard again (reversible)
Thermosetting: Heating + Moulding → Permanently set → Further heating breaks it (irreversible)
Remember: “Thermosetting” = permanently SET by heat, like how an egg hardens when cooked (can’t uncook it).
Advantages and Disadvantages of Synthetic Fibres
Advantages:
- Strong, durable, long-lasting
- Less expensive than natural fibres
- Readily available in large quantities
- Resistant to wrinkles, moths, and many chemicals
- Dry quickly (low moisture absorption)
- Available in vibrant colours that don’t fade easily
Disadvantages:
- Melt and stick to body near fire or heat — dangerous! Synthetic fabrics catch fire and melt onto skin, causing severe burns. Cotton charring is safer.
- Do not absorb sweat well — uncomfortable in hot weather
- Non-biodegradable — major environmental concern
- Causes pollution (microplastics)
Never wear synthetic clothes while working in the kitchen or near a flame. This is both a safety concept and a standard exam question: “Why should we avoid wearing synthetic clothes during cooking?”
The 4 Rs of Plastics — Environmental Concern
Plastics are non-biodegradable — microorganisms cannot break them down. They persist in soil and water for hundreds of years.
4Rs strategy:
- Reduce: Use less plastic
- Reuse: Use the same bottle multiple times
- Recycle: Process old plastic into new products
- Recover: Extract energy from plastic waste
“Biodegradable vs Non-biodegradable” is guaranteed in CBSE annual exams. Plastic bags: non-biodegradable. Vegetable peels: biodegradable. Paper: biodegradable (slowly). Give 2 examples each.
Solved Examples
Question 1: Give one example each of a thermoplastic and a thermosetting plastic. State one use of each.
Answer:
- Thermoplastic: Polyethylene (polythene) — used to make carry bags and plastic bottles.
- Thermosetting plastic: Bakelite — used to make electrical switches, handles of cooking utensils (non-conducting, heat-resistant).
Question 2: Riya left her plastic comb in a hot car. It bent out of shape. What type of plastic is the comb made of? Give reason.
Answer: The comb is made of thermoplastic. Thermoplastics soften on heating and can be deformed. When heated in the car, the comb softened and bent. On cooling, it retains the new bent shape (unless reheated and properly re-moulded).
Question 3: Why is nylon used for making parachutes?
Answer: Nylon is used for parachutes because it is: (i) Very strong — can bear the weight and air pressure (ii) Lightweight — does not add much load (iii) Elastic — can stretch without breaking (iv) Weather-resistant — unaffected by moisture or wind
Difficulty Distribution
| Level | Question Type | Example |
|---|---|---|
| Easy | Define monomer/polymer; name a thermoplastic | ~40% of questions |
| Medium | Compare rayon vs nylon; advantages of synthetic | ~40% of questions |
| Hard | Explain why synthetic clothes are dangerous; environmental impact with reasoning | ~20% of questions |
Expert Strategy
Most marks in this chapter come from comparison tables and named examples. You need to know:
- The 4 synthetic fibres (rayon, nylon, polyester, acrylic) with one use each
- Thermoplastic vs thermosetting (definition + examples)
- One fact that sets each fibre apart (nylon = first synthetic, rayon = silk-like, acrylic = wool-like)
For 3-mark “difference” questions, always write 3 points in a tabular format. Examiners award one mark per point. Don’t write long paragraphs for difference questions.
Common Traps
Trap 1: Rayon vs natural silk. Rayon is called “artificial silk” but it is a synthetic fibre, not natural silk. Rayon is made from wood pulp — but the process is chemical, so it’s synthetic (or semi-synthetic). Don’t say rayon is natural.
Trap 2: Thermoplastic vs thermosetting — students often get the examples wrong. Polythene is thermoplastic (softens on heating), Bakelite is thermosetting (does not soften). If you memorise only one example of each, let it be these two.
Trap 3: Students write that plastic is a conductor. Plastic is a very good insulator — this is why it’s used to cover electrical wires and make switches. Bakelite specifically is used because it is heat-resistant AND a good insulator.