Class 8 Science Notes Chapter 3 (Chapter 3) – Examplar Problems Book
Alright class, let's focus on Chapter 3, "Synthetic Fibres and Plastics," from your Science Exemplar book. This is a crucial chapter, not just for your school exams but also for various government exams where general science questions are common. Pay close attention to the properties, types, and environmental aspects discussed.
Chapter 3: Synthetic Fibres and Plastics - Detailed Notes for Exam Preparation
1. Introduction: Fibres
- Fibre: A thread-like strand from which textiles are made.
- Natural Fibres: Obtained from plants (e.g., cotton, jute, flax) or animals (e.g., wool, silk).
- Synthetic Fibres: Man-made fibres produced using chemical processes, mostly involving petrochemicals (raw materials derived from petroleum). Also known as artificial fibres.
2. What are Synthetic Fibres Made Of?
- Polymers: Synthetic fibres (and plastics) are polymers. A polymer is a very large molecule formed by the joining of many small repeating chemical units.
- Greek: 'Poly' means many, 'mer' means unit/part.
- Monomers: The small repeating chemical units that join together to form a polymer.
- Example: Cotton is a natural polymer called cellulose, made up of many glucose (monomer) units.
3. Types of Synthetic Fibres
-
a) Rayon (Artificial Silk):
- Source: Made from chemically treated wood pulp (cellulose), a natural source. Hence, sometimes called a semi-synthetic fibre.
- Properties: Similar texture to silk, cheaper than silk, can be dyed in various colours, absorbent (unlike most synthetics).
- Uses: Clothing (sarees, dresses), bedsheets, mixed with cotton for bedsheets or wool for carpets.
-
b) Nylon:
- Source: First fully synthetic fibre, made without any natural raw material (plant or animal). Prepared from coal, water, and air (in 1931).
- Properties: Very strong (high tensile strength), elastic, lightweight, lustrous, easy to wash, durable, water-resistant, resistant to moths and fungi.
- Uses: Ropes (including for rock climbing), tents, fishing nets, parachutes, toothbrushes, car seat belts, sleeping bags, socks, curtains.
-
c) Polyester:
- Source: Made from repeating units of a chemical called an ester. Esters are chemicals that give fruits their smell.
- Properties: Strong, lightweight, wrinkle-resistant, easy to wash and dry (dries quickly), durable, resistant to chemicals and abrasion.
- Popular Forms:
- Terylene: A popular polyester fibre, can be drawn into very fine fibres.
- PET (Polyethylene terephthalate): A familiar form of polyester used for making bottles, utensils, films, wires.
- Blends: Often blended with natural fibres:
- Polycot: Polyester + Cotton
- Polywool: Polyester + Wool
- Terrycot: Terylene + Cotton
- Uses: Dress materials, sarees, curtains, sails for boats, water hoses, conveyor belts, PET bottles and jars.
-
d) Acrylic (Artificial Wool):
- Source: Synthetic fibre made from a polymer called polyacrylonitrile.
- Properties: Resembles wool (warm, soft, lightweight), cheaper than natural wool, available in many colours, durable, resistant to moths and chemicals, easy to wash and maintain.
- Uses: Sweaters, shawls, blankets, carpets, upholstery.
4. Characteristics of Synthetic Fibres (Advantages & Disadvantages)
- Advantages:
- High strength and durability.
- Resistant to wrinkles, moths, and chemicals.
- Generally less expensive than natural fibres.
- Readily available.
- Easy to wash and dry quickly (less water absorption).
- Lightweight.
- Disadvantages:
- Melt on Heating: This is a major drawback. They melt and stick to the body if they catch fire, making them unsuitable for kitchen or laboratory work.
- Poor Sweat Absorption: Can be uncomfortable to wear in hot and humid weather.
- Non-Biodegradable: Contribute to environmental pollution as they take a very long time to decompose.
- Can cause skin allergies in some individuals.
- Generate static electricity.
5. Plastics
- Definition: Plastics are also polymers, just like synthetic fibres. They can be moulded into different shapes.
- Arrangement of Units: Monomers in plastics can be arranged in:
- Linear: Monomers linked in a straight chain.
- Cross-linked: Monomers linked in straight chains as well as crosswise, forming a network structure.
6. Types of Plastics
-
a) Thermoplastics:
- Property: Plastics that get deformed easily on heating and can be bent or remoulded into different shapes repeatedly.
- Structure: Generally linear or slightly branched polymers.
- Examples: Polythene (Poly + ethene), PVC (Polyvinyl chloride).
- Uses: Toys, combs, containers, bottles, packaging films, pipes, insulation for electric wires.
-
b) Thermosetting Plastics (Thermosets):
- Property: Plastics which, when moulded once, cannot be softened or remoulded by heating. They retain their shape even at high temperatures.
- Structure: Heavily cross-linked polymers.
- Examples:
- Bakelite: Poor conductor of heat and electricity. Used for making electrical switches, handles of various utensils (like pressure cookers, frying pans), telephone casings.
- Melamine: Resists fire and can tolerate heat better than other plastics. Used for making floor tiles, kitchenware (unbreakable dinner sets), and fire-resistant fabrics (uniforms for firefighters).
7. Plastics as Materials of Choice
- Why Preferred?
- Lightweight: Easy to handle and transport.
- Lower Price: Generally cheaper than metals.
- Good Strength and Durability: Long-lasting.
- Non-Reactive: Do not rust or corrode like iron; resistant to chemicals, air, and water. Used to store various materials, including chemicals.
- Poor Conductors: Poor conductors of heat and electricity (used as insulators, handles).
- Can be Moulded: Easily shaped into various forms.
- Can be Coloured: Available in various attractive colours.
- Specific Uses: Healthcare (syringes, gloves, medical instruments, packaging tablets), Teflon coating (non-stick cookware), Microwave-safe plastic cookware, Packaging (food, water, milk, etc.).
8. Plastics and the Environment
- Biodegradable: Materials that get decomposed through natural processes (like bacterial action) in a relatively short time (e.g., paper, cotton cloth, wood, vegetable peels).
- Non-Biodegradable: Materials that are not easily decomposed by natural processes and persist in the environment for a very long time (e.g., plastics, tin cans, glass).
- Environmental Problems Caused by Plastics:
- Pollution: Accumulate in landfills, pollute water bodies, block drains.
- Harm to Animals: Animals may ingest plastic bags, leading to death. Stray animals often eat garbage containing plastic wrappers.
- Air Pollution: Burning plastics releases poisonous fumes into the atmosphere.
- Solutions - The 5 R Principle:
- Reduce: Minimize the use of plastic items (e.g., carry cloth/jute bags for shopping).
- Reuse: Use plastic containers or bottles multiple times for storage or other purposes.
- Recycle: Segregate plastic waste so it can be melted and remoulded into new items (Note: Not all plastics are easily recyclable; thermosetting plastics are difficult). Colouring agents added during recycling limit usage, especially for food storage.
- Recover: Convert waste into resources (e.g., generating energy from waste).
- Refuse: Avoid using single-use plastics.
- Responsible Citizen Actions: Do not throw plastic bags on roads or in water bodies. Use cotton or jute bags. Segregate biodegradable and non-biodegradable waste before disposal.
Multiple Choice Questions (MCQs)
-
Which of the following is considered the first fully synthetic fibre, made without using natural raw materials?
a) Rayon
b) Nylon
c) Polyester
d) Acrylic -
Bakelite and Melamine are examples of:
a) Thermoplastics
b) Thermosetting plastics
c) Natural polymers
d) Elastomers -
The monomer unit of cellulose (found in cotton and wood pulp) is:
a) Ester
b) Ethene
c) Glucose
d) Amino acid -
Which synthetic fibre is known as 'artificial silk' and is derived from wood pulp?
a) Nylon
b) Acrylic
c) Terylene
d) Rayon -
PET, commonly used for making bottles and jars, is a type of:
a) Nylon
b) Rayon
c) Polyester
d) Acrylic -
Which property makes Melamine suitable for making floor tiles and kitchenware?
a) It is a very good conductor of heat.
b) It is very soft and flexible.
c) It resists fire and tolerates heat well.
d) It is transparent like glass. -
A major environmental concern regarding plastics is that they are mostly:
a) Biodegradable
b) Very expensive
c) Good conductors of electricity
d) Non-biodegradable -
Which of the following is NOT an advantage of synthetic fibres?
a) They dry up quickly.
b) They are durable.
c) They are less expensive.
d) They absorb sweat well. -
The '5 R Principle' is advocated for managing plastic waste. Which of the following is NOT one of the 'R's?
a) Reduce
b) Reuse
c) Remove
d) Recycle -
Why is it advised not to wear synthetic clothes while working in the kitchen or laboratory?
a) They absorb too much water.
b) They catch fire easily and melt, sticking to the body.
c) They are poor conductors of heat.
d) They are too expensive to risk damaging.
Answer Key for MCQs:
- b) Nylon
- b) Thermosetting plastics
- c) Glucose
- d) Rayon
- c) Polyester
- c) It resists fire and tolerates heat well.
- d) Non-biodegradable
- d) They absorb sweat well. (This is a disadvantage)
- c) Remove (The principles are Reduce, Reuse, Recycle, Recover, Refuse)
- b) They catch fire easily and melt, sticking to the body.
Study these notes thoroughly. Remember the specific examples, properties, and uses of each fibre and plastic type, as well as the environmental impact. Good luck with your preparation!