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 detailed explanations and answers to exercises covering the nature of synthetic fibres, their production from petrochemicals, and the concept of polymers. It differentiates between various types of synthetic fibres like rayon, nylon, and acrylic, and explores the properties and applications of plastics, including thermoplastics and thermosetting plastics. The solutions also highlight the environmental impact of plastics and ways to manage them. These solutions are designed to help students grasp the fundamental concepts, understand the differences between natural and synthetic materials, and prepare effectively for their exams by offering clear, step-by-step explanations.
Quick info
| Board | CBSE |
|---|---|
| Class | Class 8 |
| Subject | Science |
| Session | 2026 |
| Language | English |
| Type | NCERT Solutions |
| Chapter | Chapter 3: Synthetic Fibres and Plastics |
Chapter summary
Chapter 3 of the CBSE Class 8 Science curriculum focuses on Synthetic Fibres and Plastics. The NCERT Solutions cover the definition and types of synthetic fibres, explaining that they are man-made polymers derived from petrochemicals. Key concepts like polymers, thermoplastics, and thermosetting plastics are explained with examples and their distinct properties. The solutions also touch upon the widespread use of plastics and their environmental implications, guiding students to understand these materials better.
Learning outcomes
- Understand the definition and origin of synthetic fibres.
- Differentiate between natural and synthetic fibres.
- Explain the concept of polymers and their role in synthetic materials.
- Identify and differentiate between thermoplastics and thermosetting plastics.
- Recognize the common uses and properties of various synthetic fibres and plastics.
- Appreciate the environmental concerns related to plastic usage.
Topics covered
Paper topics
- Synthetic Fibres
- Man-made Fibres
- Polymers
- Petrochemicals
- Rayon
- Nylon
- Acrylic
- Plastics
- Thermoplastics
- Thermosetting Plastics
- Properties of Plastics
- Uses of Plastics
Important topics
- Definition and types of synthetic fibres
- Concept of polymers
- Difference between thermoplastics and thermosetting plastics
- Properties and uses of nylon
- Applications of plastics in daily life
PDF preview
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Questions and Solutions
Question 1
Fibres are classified as natural or synthetic. Synthetic fibres are called so because they are not derived from natural sources like plants or animals. Instead, they are manufactured through complex chemical processes, typically using raw materials obtained from petrochemicals. These fibres are essentially long chains of small repeating units called polymers. Examples of synthetic fibres include rayon, nylon, and acrylic.
Question 2
The correct answer is (b) it is obtained from wood pulp.
While most synthetic fibres are entirely man-made from chemicals, rayon is a unique case. It is manufactured through a chemical process, but its primary raw material is cellulose, which is obtained from wood pulp, a natural source. This makes rayon a 'regenerated' fibre, distinct from purely synthetic ones like nylon or polyester.
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 remarkably strong, lightweight, and elastic. Their high tensile strength is evident from their widespread use in demanding applications. For instance, nylon is used to manufacture parachutes because it can withstand significant force and is resistant to tearing. It is also used for making ropes for rock climbing, which must be incredibly strong to support a person's weight. Additionally, nylon is used in fishing nets due to its durability and strength in water, and in seat belts in vehicles to provide safety in case of sudden stops or collisions.
Question 5
Plastic containers are frequently chosen for storing food due to several advantageous properties:
- Lightweight: They are much lighter than containers made of glass or metal, making them easier to handle, transport, and store.
- Affordability: Plastic containers are generally less expensive to produce and purchase compared to alternatives like glass or stainless steel.
- Good Strength: Despite being lightweight, many plastics possess good structural integrity and are resistant to breaking.
- Durability: They are long-lasting and can withstand regular use without significant degradation.
- Non-reactive: Most food-grade plastics do not react with food items, preventing contamination and preserving the taste and quality of the food.
Question 6
The key differences between thermoplastic and thermosetting plastics lie in their behaviour upon heating and their molecular structure:
| Feature | Thermoplastics | Thermosetting Plastics |
|---|---|---|
| Softening on Heating | They soften easily when heated and can be moulded into different shapes. They can be remoulded multiple times. | They do not soften on heating. Once moulded, they undergo irreversible chemical changes and cannot be softened or remoulded. |
| Molecular Structure | Consist of long, linear or branched polymer chains with weak intermolecular forces. | Have a three-dimensional network structure formed by cross-linking between polymer chains, creating strong bonds. |
| Examples | Polythene (used for making toys, bags, combs), PVC (used for pipes, footwear). | Bakelite (used for electrical switches, handles of utensils), Melamine (used for floor tiles, kitchenware, fire-resistant fabrics). |
| Applications | Manufacturing of toys, containers, household items, pipes. | Electrical appliances, handles of cooking utensils, car parts, fire-resistant materials. |
Common mistakes
- Confusing rayon as a purely synthetic fibre instead of a regenerated one.
- Not clearly distinguishing between the properties and applications of thermoplastics and thermosetting plastics.
- Overlooking the role of polymers as the basic building blocks of synthetic fibres and plastics.
Revision tips
- Focus on understanding the 'why' behind the properties of synthetic materials, not just memorizing them.
- Create a table to compare and contrast thermoplastics and thermosetting plastics.
- Relate the examples given in the solutions to everyday objects made from synthetic fibres and plastics.
- Pay attention to the definitions of key terms like 'polymer' and 'monomer'.
Practice MCQs
Q1. Which of the following is a key characteristic that distinguishes rayon from other synthetic fibres?
Explanation: Rayon is unique because it is derived from natural sources (wood pulp) through chemical processing, unlike purely synthetic fibres which are entirely man-made from chemicals.
Q2. What is the primary raw material used for synthesizing most synthetic fibres?
Explanation: Synthetic fibres are chemically synthesized from raw materials derived from petrochemicals, which are products of petroleum.
Q3. Which property makes nylon fibres suitable for making parachutes and climbing ropes?
Explanation: Nylon fibres are known for their exceptional strength, making them ideal for applications that require high tensile strength and durability, such as parachutes and climbing ropes.
Q4. Which type of plastic can be softened by heating and moulded again?
Explanation: Thermoplastics are plastics that can be repeatedly softened by heating and then moulded into different shapes, making them versatile for manufacturing.
Q5. Bakelite is an example of which type of plastic?
Explanation: Bakelite is a thermosetting plastic, meaning it undergoes irreversible chemical changes upon heating and cannot be remoulded once set.
Frequently asked questions
What are synthetic fibres?
Synthetic fibres are man-made fibres, not obtained from natural sources like plants or animals. They are synthesized from raw materials called petrochemicals and are essentially polymers.
Why is rayon considered different from other synthetic fibres?
Rayon is considered different because, although it is man-made, it is derived from natural sources, specifically wood pulp, through chemical processing. 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. Synthetic fibres and plastics are examples of polymers.
What is the main difference between thermoplastics and thermosetting plastics?
Thermoplastics can be softened by heating and remoulded multiple times, while thermosetting plastics undergo irreversible changes upon heating and cannot be softened or remoulded.
Why are nylon fibres considered very strong?
Nylon fibres are exceptionally strong, elastic, and lightweight. This strength makes them suitable for applications like parachutes, climbing ropes, and seat belts, where durability and resistance to breaking are crucial.
Are plastic containers safe for storing food?
Plastic containers are often favoured for storing food due to their light weight, low cost, durability, and resistance to corrosion. However, it's important to use food-grade plastics specifically designed for this purpose.
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