Lakhmir Singh Solutions for Class 8 Science Chapter 3 – Synthetic Fibres and Plastics

Lakhmir Singh Solutions for Class 8 Science Chapter 3 – Synthetic Fibres and Plastics offer a clear and engaging way to explore one of the most practical topics in everyday life. This chapter helps students understand the science behind materials we use all around us—like nylon ropes, plastic containers, polyester clothes, and acrylic sweaters. With simple explanations and real-world examples, the solutions guide learners through the fascinating journey of how synthetic fibres are made and why plastics have become so widely used.

Designed to align with the NCERT Class 8 Science curriculum, these solutions make complex concepts feel easy. Students will learn about different types of synthetic fibres such as rayon, nylon, polyester, and acrylic, their properties, and how they differ from natural fibres. The chapter also explains the impact of plastics on the environment and introduces the concept of biodegradability—a key idea in today’s eco-conscious world.

Each answer in Lakhmir Singh’s Science book for Chapter 3 is structured to improve understanding and retention. Whether it’s learning about thermoplastics vs. thermosetting plastics, or understanding why synthetic fibres are strong, lightweight, and water-resistant, the step-by-step solutions offer clear, student-friendly guidance.

One of the best things about using Lakhmir Singh Solutions for Chapter 3 is that they don’t just help with homework—they build a strong foundation in science. These solutions include important definitions, solved exercises, diagrams, and extra tips to boost exam preparation. From school assessments to Olympiad-level questions, they help students stay ahead.

So if you're a student aiming to score well or simply curious about the materials shaping our modern world, Class 8 Science Chapter 3 Synthetic Fibres and Plastics will open your eyes to how science makes our lives more convenient—and why we must also be responsible users of synthetic materials.

Download Lakhmir Singh Solutions for Class 8 Science Chapter 3 – Synthetic Fibres and Plastics PDF Here

The Lakhmir Singh Solutions for Chapter 3 on Synthetic Fibres and Plastics provide detailed explanations and answers to help you master this important topic.
This chapter covers essential concepts about man-made materials that have revolutionized our daily lives. You'll learn about different types of synthetic fibres like rayon, nylon, polyester, and acrylic, as well as various plastics and their properties. The solutions guide includes detailed answers to all textbook questions, helping you understand the manufacturing processes, characteristics, and environmental impacts of these materials.

Download the Lakhmir Singh Solutions PDF today to improve your understanding of synthetic materials and boost your performance in Science class.

Access Answers to Lakhmir Singh Solutions for Class 8 Science Chapter 3

1. How does cellulose transform into rayon during manufacturing?
   Answer: During rayon production, wood pulp cellulose undergoes chemical treatment with sodium hydroxide to create soda cellulose. This is then treated with carbon disulfide to form cellulose xanthate, which is dissolved in dilute sodium hydroxide. The solution is finally extruded through tiny holes into an acid bath, where it solidifies into rayon fibers.
2. Why doesn't a bakelite electric switch deform during a short circuit, while a polythene container warps in hot water?
   Answer: Bakelite is a thermosetting plastic with extensive cross-linked polymer chains that create a rigid three-dimensional network structure. These chemical bonds cannot be broken by heat alone, preventing deformation during short circuits. Polythene, being a thermoplastic, has linear polymer chains held together by weak intermolecular forces that easily separate when heated, causing it to warp in hot water.
3. What makes acrylic suitable as a wool substitute compared to other synthetic fibers?
   Answer: Acrylic mimics wool better than other synthetics because it contains fibers with tiny air pockets that provide similar insulation properties. Its polymer structure allows for crimping that resembles wool's natural texture. Additionally, acrylic can be engineered to have a soft, warm feel while remaining lightweight and resistant to moths and mildew that typically damage natural wool.
4. How does the chemical structure of PET contribute to its widespread use in beverage bottles?
   Answer: PET's chemical structure includes rigid aromatic rings from terephthalic acid combined with flexible ethylene glycol segments. This creates a semi-crystalline polymer with excellent barrier properties against gases (preventing carbonation loss), good transparency, high strength-to-weight ratio, and sufficient heat resistance to withstand hot-fill processes, making it ideal for beverage bottles.
5. Why are burn injuries from synthetic clothing often more severe than those from natural fibers?
   Answer: Synthetic fibers like polyester and nylon melt rather than char when burned, creating a molten material that adheres to skin and continues transferring heat, causing deeper, more concentrated burns. Natural fibers typically char and ash away instead of melting, distributing heat less intensely and creating a thermal barrier that can reduce injury severity.
6. Explain the paradox of nylon being both stronger than steel wire yet unsuitable for high-heat applications.
   Answer: Nylon's paradoxical properties stem from its molecular structure. Its strong hydrogen bonds and orderly crystalline regions create exceptional tensile strength (stronger per weight than steel). However, these same molecular chains have relatively weak intermolecular forces that break at relatively low temperatures (220-260°C), causing it to melt quickly, unlike steel's metallic bonds that remain stable at much higher temperatures.
7. How do esters in polyester influence its fabric properties compared to polyamides in nylon?
   Answer: The ester linkages in polyester make it more resistant to hydrolysis and chemical degradation than the amide bonds in nylon. This gives polyester better color retention, wrinkle resistance, and dimensional stability. However, these same ester groups create a more hydrophobic fiber with lower moisture absorption (0.4% vs. nylon's 4.5%), making polyester less comfortable against skin but better for moisture-wicking applications.
8. What properties make melamine suitable for both kitchenware and firefighter uniforms?
   Answer: Melamine's triazine ring structure with multiple nitrogen atoms creates an extremely stable, thermally resistant network when polymerized. This structure provides exceptional heat resistance (withstanding temperatures up to 150°C without degradation), flame retardancy (doesn't readily ignite), excellent hardness, and resistance to chemicals and stains. These properties make it suitable for both heat-exposed kitchenware and protective firefighting gear.
9. How do the disposal challenges differ between thermoplastic and thermosetting waste?
   Answer: Thermoplastics can be repeatedly melted and reformed, making them theoretically recyclable through mechanical processes. In contrast, thermosetting plastics cannot be remelted once formed due to their permanent cross-linked structure, making conventional recycling impossible. Thermosetting waste typically requires energy recovery through incineration or chemical recycling techniques that break down the polymer into base chemicals, presenting greater disposal challenges.
10. Why doesn't Teflon stick to food despite being a plastic material?
    Answer: Teflon (polytetrafluoroethylene) resists adhesion because its carbon backbone is surrounded by fluorine atoms that create an extremely stable, chemically inert surface with very low surface energy (similar to how oil repels water). The fluorine atoms form a tightly packed shield around the carbon chain, preventing the formation of van der Waals forces or hydrogen bonds with food molecules that would cause sticking.
11. How does PVC transition from rigid pipe material to flexible medical tubing?
    Answer: Pure PVC is naturally rigid due to strong interactions between its chlorine atoms. To create flexible PVC, manufacturers add plasticizers (typically phthalate compounds) that position themselves between polymer chains, disrupting these interactions and allowing the chains to slide past each other. By varying the plasticizer concentration (from 0% in rigid pipes to over 40% in medical tubing), manufacturers can precisely control PVC's flexibility for different applications.
12. What makes thermosetting plastics suitable for electrical applications despite their recycling limitations?
    Answer: Thermosetting plastics excel in electrical applications because their cross-linked structure provides exceptional electrical insulation properties that remain stable across temperature fluctuations. They resist electrical tracking (formation of conductive carbon paths), have high arc resistance, maintain dimensional stability under heat, and don't soften when components generate heat. These safety-critical properties outweigh recycling limitations in electrical applications where failure could cause fires or electrocution.
13. Why can acrylic fabric trigger more static electricity than cotton?
    Answer: Acrylic generates more static electricity than cotton because its polymer structure lacks the hydroxyl groups found in cotton's cellulose molecules. Without these groups, acrylic cannot absorb moisture from the air (moisture content <2% versus cotton's 8-10%), resulting in poor electrical conductivity.

Conceptual Based Questions

Question 1:Compare the environmental impact of producing rayon versus fully synthetic fibers like nylon. Which process contributes more to deforestation and why?
Answer: Rayon production has a greater negative impact on forests compared to fully synthetic fibers like nylon. Rayon is manufactured by chemically treating wood pulp or cellulose, which requires harvesting trees, directly contributing to deforestation. In contrast, nylon and other fully synthetic fibers are produced from petrochemicals derived from coal, water, and air, which don't directly require forest resources. While petrochemical production has its own environmental concerns, it doesn't directly drive deforestation the way rayon production does.

Question 2: Compare and contrast PET plastic used in bottles with PVC used in pipes in terms of their chemical structure, properties, and environmental impact upon disposal.
Answer:
Chemical Structure:

• PET (Polyethylene Terephthalate): Contains ester linkages in its polymer backbone, formed by the reaction between terephthalic acid and ethylene glycol.
• PVC (Polyvinyl Chloride): Contains carbon-carbon bonds with chlorine atoms attached to every other carbon atom.

Properties:

• PET: Transparent, lightweight, strong, and has excellent barrier properties against gases. It's relatively heat-resistant and maintains its shape well.
• PVC: Rigid in its pure form, but can be made flexible with plasticizers. It's durable, resistant to chemical corrosion, and has good electrical insulation properties.

Environmental Impact:

• PET: More easily recycled than many plastics (recycle code #1) and doesn't release harmful chemicals when landfilled. However, it persists in the environment for hundreds of years if not recycled.
• PVC: More problematic environmentally. When incinerated, it can release hydrochloric acid and potentially dioxins. It contains chlorine, which presents additional disposal challenges, and the plasticizers used in flexible PVC can leach out over time. It's less commonly recycled (recycle code #3).

Both are thermoplastics that can theoretically be melted and reformed, though PET is more commonly recycled in practice due to infrastructure and economic factors.

Application Based Question

Q1. A manufacturing company is deciding between using thermoplastic or thermosetting plastic for electrical outlet covers. Develop an argument for which type would be most appropriate based on their properties and the specific application requirements.
Answer:
Electrical outlet covers should be made from thermosetting plastics like Bakelite or melamine for several critical reasons:

1. Electrical safety: Thermosetting plastics are excellent electrical insulators and maintain this property consistently over time and varying conditions. This is essential for preventing electrical leakage and potential shock hazards.
2. Heat resistance: Unlike thermoplastics, thermosetting plastics will not soften or deform when exposed to heat generated by electrical currents or environmental factors. This prevents warping that could expose live electrical components.
3. Dimensional stability: Once molded, thermosetting plastics maintain their shape permanently, ensuring that the outlet cover continues to fit properly and provide complete coverage of the electrical components.
4. Chemical resistance: Outlet covers may be exposed to cleaning agents and other household chemicals. Thermosetting plastics generally have superior chemical resistance compared to many thermoplastics.
5. Durability: The cross-linked structure of thermosetting plastics provides excellent mechanical strength and impact resistance, reducing the likelihood of cracks or breaks that could compromise electrical safety.

While thermoplastics might offer advantages in terms of recyclability and potentially lower production costs, the safety requirements of electrical applications make thermosetting plastics the clear choice for outlet covers.