CBSE Class 8 Science Chapter 3: Synthetic Fibres and Plastics NCERT Solutions
This chapter delves into the world of synthetic fibres and plastics, crucial materials in our daily lives. The NCERT Solutions for Class 8 Science, Chapter 3, provide clear explanations and step-by-step solutions to the exercises. Students will learn about the difference between natural and synthetic fibres, the properties of various synthetic materials like rayon, nylon, and polyester, and the concept of polymers. The solutions also cover the characteristics of plastics, including thermoplastics and thermosetting plastics, and their diverse applications. Understanding these concepts is vital for comprehending the materials science around us and for performing well in exams. These solutions aim to simplify complex topics, making revision effective and efficient for students.
Quick info
| Board | CBSE |
|---|---|
| Class | Class 8 |
| Subject | Science |
| Session | 2026 |
| Language | English |
| Type | NCERT Solutions |
| Chapter | Chapter 3 |
Chapter summary
Chapter 3 of the CBSE Class 8 Science syllabus focuses on Synthetic Fibres and Plastics. The NCERT Solutions cover the distinction between natural and synthetic fibres, the process of creating synthetic fibres from raw materials like petrochemicals, and the properties of polymers. It also explains the characteristics and applications of different types of plastics, namely thermoplastics and thermosetting plastics, highlighting their differences and uses in everyday objects. The exercises are designed to reinforce these concepts.
Learning outcomes
- Understand the difference between natural and synthetic fibres.
- Identify the raw materials used to synthesize synthetic fibres.
- Explain the concept of polymers and their formation.
- Differentiate between thermoplastics and thermosetting plastics.
- List the properties and applications of various synthetic fibres and plastics.
Topics covered
Paper topics
- Synthetic Fibres
- Natural Fibres
- Man-made Fibres
- Polymers
- Petrochemicals
- Rayon
- Nylon
- Polyester
- Plastics
- Thermoplastics
- Thermosetting Plastics
- Applications of Synthetic Materials
Important topics
- Difference between natural and synthetic fibres
- Properties and uses of Nylon
- Concept of Polymers
- Thermoplastics vs. Thermosetting Plastics
- Applications of Plastics
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Questions and Solutions
Question 1
Some fibres are termed 'synthetic' because they are not derived from natural sources like plants or animals. Instead, they are manufactured through chemical processes, often using petrochemicals as the raw material. These fibres are essentially large molecules called polymers, formed by joining together many smaller repeating units. Examples of synthetic fibres include rayon, nylon, and polyester.
Question 2
The correct answer is (b) it is obtained from wood pulp.
While rayon is a man-made fibre, it is produced from cellulose, which is obtained from wood pulp. This gives it a semi-synthetic nature, distinguishing it from purely synthetic fibres like nylon or polyester, which are synthesized entirely from chemicals derived from petrochemicals.
Question 3
(a) Synthetic fibres are also called _____ or ____ fibres.
(b) Synthetic fibres are synthesised from raw material called _____.
(c) Like synthetic fibres, plastic is also a ______.
(a) Synthetic fibres are also called man-made or artificial fibres.
(b) Synthetic fibres are synthesised from raw material called petrochemicals.
(c) Like synthetic fibres, plastic is also a polymer.
Question 4
Nylon fibres are exceptionally strong, which is why they are used in applications requiring high tensile strength and durability. For instance, nylon is used to manufacture parachutes because it can withstand significant force and is lightweight. It is also used for making ropes for rock climbing, which must be strong enough to support a person's weight. Additionally, nylon is utilized in fishing nets and seat belts in cars, demonstrating its robust nature and ability to resist breaking under stress.
Question 5
Plastic containers are widely preferred for storing food due to several advantageous properties:
- Lightweight: They are much lighter than containers made of glass or metal, making them easy to handle and transport.
- Affordability: Plastic containers are generally less expensive to produce and purchase compared to alternatives.
- Good Strength: They possess adequate strength to hold food items without easily breaking or deforming.
- Durability: Plastic is a durable material that can last for a long time, resisting breakage and corrosion.
- Non-reactive: Many plastics do not react with food substances, preventing contamination and preserving the taste and quality of the food.
Question 6
The key differences between thermoplastic and thermosetting plastics are as follows:
| Feature | Thermoplastic | Thermosetting Plastic |
|---|---|---|
| Behavior on Heating | Gets easily deformed and softened upon heating; can be molded repeatedly. | Cannot be softened by heating once molded; they undergo irreversible chemical changes. |
| Structure | Composed of long, linear or branched polymer chains with weak intermolecular forces. | Form a rigid, cross-linked network structure due to strong intermolecular forces. |
| Examples | Polythene (used for toys, containers), PVC (used for pipes, bottles). | Bakelite (used for electrical switches, handles), Melamine (used for floor tiles, kitchenware). |
| Recyclability | Generally recyclable as they can be melted and remolded. | Not easily recyclable as they do not melt upon heating. |
Common mistakes
- Confusing rayon as a purely synthetic fibre.
- Not clearly distinguishing between thermoplastic and thermosetting plastics.
- Overlooking the strength and elasticity of nylon.
- Failing to recall the raw materials for synthetic fibres.
Revision tips
- Create a table comparing the properties and uses of different synthetic fibres.
- Draw diagrams to illustrate the difference between thermoplastic and thermosetting plastics.
- Focus on understanding the 'why' behind the applications of these materials.
- Practice identifying examples of synthetic fibres and plastics in your surroundings.
Practice MCQs
Q1. Which of the following is a key characteristic that distinguishes rayon from other synthetic fibres?
Explanation: Rayon is unique because, unlike other synthetic fibres derived from petrochemicals, it is manufactured from cellulose obtained from wood pulp, giving it a natural origin component.
Q2. What is the primary raw material used for synthesizing most synthetic fibres?
Explanation: Most synthetic fibres are produced through chemical processing of petrochemicals, which are derived from petroleum.
Q3. Which type of plastic can be softened by heating and can be easily bent?
Explanation: Thermoplastics are plastics that soften upon heating and can be molded repeatedly, making them easy to bend and reshape.
Q4. Nylon fibres are often compared to steel wire for which property?
Explanation: Nylon is known for its exceptional strength, with nylon threads being stronger than steel wires of the same thickness.
Q5. Which of the following is an example of a thermosetting plastic?
Explanation: Bakelite is a thermosetting plastic, meaning it hardens permanently upon heating and cannot be softened again, making it suitable for electrical switches and handles.
Frequently asked questions
What are synthetic fibres?
Synthetic fibres are man-made fibres that are synthesized from raw materials, primarily petrochemicals, through chemical processes. Examples include nylon, polyester, and rayon.
Why is rayon considered different from other synthetic fibres?
Rayon is different because it is derived from wood pulp (cellulose), which is a natural source, although it undergoes chemical processing to form fibres. Other synthetic fibres are entirely synthesized from chemicals.
What is a polymer?
A polymer is a large molecule made up of many small, repeating units called monomers, joined together. Both synthetic fibres and plastics are polymers.
What is the main difference between thermoplastics and thermosetting plastics?
Thermoplastics can be softened by heating and reshaped multiple times, while thermosetting plastics undergo irreversible chemical changes upon heating, hardening permanently and cannot be reshaped.
Why are plastic containers popular for storing food?
Plastic containers are favoured for food storage because they are lightweight, relatively inexpensive, durable, and often resistant to corrosion, making them easy to handle and long-lasting.
Give examples of the uses of nylon.
Nylon's strength makes it ideal for applications like parachutes, rock climbing ropes, fishing nets, and seat belts in vehicles.
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