Important Questions Class 8 Science Chapter 3

Important Questions Class 8 Science Chapter 3 – Synthetic Fibres and Plastics

Science as a subject plays a vital role in a student’s academic life. Class 8 is one of the stepping stones for the students; therefore, learning the basics of Science and comprehending the theory and concepts introduced in the chapters is essential. Studying and practising the subject is important; many students enjoy learning and understanding the Science subject. 

This chapter includes different topics like synthetic fibres and their meaning, types of synthetic fibres, the characteristics, plastics, various benefits of plastics, use of plastic as a choice, hazards of use of plastic on our environment, and exposure to many other fibres like nylon, rayon, melamine, bakelite, etc. The chapter mainly deals with the type of cloth we utilise daily. These clothes are made up of fabric, so two types of fabric are explained in the chapter. The natural fibre is obtained from plants, and humans synthesise synthetic fibre for different purposes using some chemicals.

 Important topics that are covered in Chapter 3 of Class 8 Science – Synthetic Fibres and Plastics:

• What are synthetic fibres?
• The types of synthetic fibres
• Characteristics of these synthetic fibres
• Plastics
• Plastics as materials of choice
• Plastics and impact on the environment

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A few students might face difficulty in understanding the chapters, as they find it difficult to memorise concepts and names in the answers. Therefore, practising questions and answers from Extramarks question bank Important Questions Class 8 Science Chapter 3 will benefit students massively by providing them stepwise solutions to every question.

Science Class 8 Chapter 3 important questions covers all aspects of the chapter and bring a complete set of questions per the latest CBSE syllabus. Our subject matter experts at Extramarks have compiled important questions Class 8 Science Chapter 3 from various reference materials: CBSE past year question papers, CBSE sample paper and NCERT textbook. Students will also get comprehensive solutions for each type of question. Important questions Class 8 Science Chapter 3 includes long answer types, short answer types, and MCQs.

Important Questions Class 8 Science Chapter 3 – With Solutions

You can also find CBSE Class 8 Science Chapter-by-Chapter Important Questions here:

Question 1. Which of the following groups contains all synthetic substances?

• Nylon, Terylene, Wool
• Cotton, Polycot, Rayon
• PVC, Polythene, Bakelite
• Acrylic, Silk, Wool

Answer 1: The correct option is C.

Explanation: 

Polymer is a large molecule with very high molecular mass and repeated structural units called monomers. The process included in the formation of polymer from its distinct monomer is called polymerization. They can be classified as natural, synthetic and semi-synthetic polymers. Synthetic polymers are substances which are man-made, derived from petroleum oils and extensively used in daily life and industry.

• Nylon was the first commercially used successful synthetic thermoplastic polymer.
• Terylene or polyethene terephthalate or polyester is for synthetic fibres.
• A blend of wool and kevlar is the synthetic fibre widely used in body armour, was lighter, cheaper and worked better in damp conditions than kevlar alone. Moreover, wool is extracted from an animal source.
• Cotton is a soft, fluffy staple fibre that grows in a boll, or protective capsule, around the seeds of cotton plants. Cotton is derived from plant sources.
• The fibre is almost pure cellulose. So it is not synthetic.
• Rayon is derived from natural cellulose and cannot be considered synthetic.
• PVC, polyethene, and bakelite are three synthetic polymers.

Question 2. Explain why the following are made of thermosetting plastics.

                     (a) Saucepan handles 

                     (b) Electric plugs/switches/plug boards

Answer 2: 

Thermosetting plastics:

• These are types of plastics that do not get deformed or softened on heating when moulded once.
• Thermosetting plastics are made up of long cross-linked chains of molecules.
• The main characteristics of these plastics are the poor conductivity of heat and electricity.
• They are made up of very rigid structures.
• The main examples of thermosetting plastics are bakelite and melamine.

Reasons:

• Thermosetting plastics are used in saucepan handles because they do not soften on heating and cannot be bent easily. These plastics, like bakelite, are poor conductors of heat.
• Thermosetting plastics are generally used in electric plugs/switches/plug boards as they are bad conductors of electricity. They do not get moulded. Also, they are hard and strong.
• Bakelite is also a thermosetting plastic that is a poor conductor of heat and electricity. 

Question 3. Write about the importance of synthetic polymers in our life.​

Answer 3: 

Synthetic polymers are substances that are man-made, derived from petroleum oils and extensively used in daily life as well as industry. Synthetic fibres are cheap and thus are feasible for everyone.

Various synthetic polymers are used in our daily life. Some of the major uses of these polymers are as follows-

• Nylon is used for making ropes for rock climbing, fishing nets, raincoats, parachutes and tyre cords, etc., due to its excellent elasticity and high strength.
• Acrylic is used for sweaters, tracksuits, linings for boots and gloves and In furnishing fabrics and various carpets because of its similarity with many natural fibres.
• Terylene- It is used in the textile industry for making clothes like sarees, tapestry and dress material. It is mixed with natural fibre like cotton and wool to make a vast variety of clothes.
• Rayon- It is used for making carpets, tyre cords, and surgical dressings in the textile industry.
• Plastics- It is used to store food items, water, milk, pickles, dry food, etc. Plastic containers seem to be the most convenient. This broad usage is because of its lightweight, low price, good strength and easy handling compared to metals. Plastics are used in cars, air crafts and spacecraft.

Question 4. Despite being very useful, we are advised to restrict the use of plastic. Why is it so? Can you suggest some methods to limit its consumption?

Answer 4: 

Plastic is a non-biodegradable material, and as such, it causes land pollution. At the same time, burning materials like plastic as garbage causes serious air pollution.

Improper disposal of plastics causes several problems. Some of these are:

• The littering of plastics in open and public spaces creates unhygienic conditions, as it acts as a major breeding ground for insects and mosquitoes that cause diseases like malaria and dengue.
• Plastics do not undergo degradation; thus, they stay in the soil for many years, which affects soil fertility and degrades the soil quality.
• Plastic blocks the pipes if it enters the drainage and sewerage system and the drains causing water logging.
• Plastic items often find their way to the river and other water bodies, which fish and seabirds then swallow, and other marine species, thus leading to suffocation and death.

Some ways to limit its consumption are:

• Paper bags are highly recommended to use instead of plastics.
• Reuse if it is possible to reduce plastic consumption, thereby decreasing its consumption.
• Recycling of plastic. It requires the plastic to be collected, sorted, chopped, melted and remoulded.
• Do not burn the plastic as it causes the release of carbon monoxide, which is a very harmful gas-causing cancer.

Question 5. The most suitable material for the preparation of handles of cooking utensils is _______.

• polythene
• nylon
• PVC
• bakelite

Answer 5: The correct option is D.

Explanation:

Polythene is not used for the preparation of handles of cooking utensils since it gets soft on heating. Therefore, this option gets eliminated.

PVC is polyvinyl chloride, a thermoplastic, and it gets melted on heating, due to which it is not used for the preparation of cooking utensils. Thus, this option gets eliminated.

Option (D) is correct: For making the handles of cooking utensils, the most suitable material is bakelite. Bakelite is a very poor conductor of heat and does not become soft on getting heated, so it is the preferred material for making the handles of many cooking utensils. Bakelite is the commercial name kept for the polymer material obtained by the polymerization of phenol and formaldehyde.

Question 6. ‘Manufacturing synthetic fibres are actually helping conservation of forest’. Comment.

Answer 6: 

Synthetic fibres are made from synthesised polymers of small molecules. The compounds used to make these fibres come from various raw materials such as petroleum-based chemicals or petrochemicals. These materials are also polymerized into a chemical that bonds two adjacent carbon atoms. Differing chemical compounds are mainly used to produce different types of synthetic fibres. Synthetic fibres are made only from natural gas polymers and petroleum by-products.

Manufacturing synthetic fibres are actually helping the conservation of forests. Synthetic fibres are man-made fibres, and they are also manufactured from petrochemicals. If we use these synthetic fibres, we do not require to cut trees down and hence help in the conservation of forests.

Since synthetic fibres are mainly obtained from petrochemicals, they have excellently substituted the use of different natural fibres like cotton, wool, silk and jute, which are obtained from animals and plants. As a result, plants need not be cut to produce fibres.

Synthetic fibres are considered more durable than most natural fibres and readily pick up different dyes. In addition, many synthetic fibres offer consumer-friendly functions like stretching, waterproofing and also stain resistant. Sunlight, moisture, and oils produced by the human skin cause all fibres to break down easily and wear away.

Plant fibres are obtained from different parts of plants, such as the seeds (cotton, milkweed, kapok), stems (jute, hemp, flax, ramie, kenaf, nettle, kenaf, bamboo), and leaves (manila, sisal, abaca), fruit (coir) and various other grass fibres. Fibres from these plants are considered to be renewable and biodegradable.

The manufacturing of synthetic fibres aids forest conservation since natural fibres need the extraction of raw materials from plants, which necessitates the falling of many trees. This proves to be very beneficial in forest conservation.

 Some synthetic fibres are listed below.

• Rayon
• Nylon
• Polyester

Advantages of Synthetic Fibres

• Synthetic fibres are extremely durable and do not wrinkle easily.
• They are elastic and are easily stretched out.
• They are very strong and can sustain a heavy load.
• They are soft and hence used in clothing materials.
• They are cheaper than natural fibres.

Question 7. ‘Avoid plastics as far as possible’. Comment on this advice.

Answer 7: 

Plastic manufacturing involves the addition of potentially hazardous compounds like stabilisers or colourants. Many of them are not even been subjected to environmental risk assessments. Therefore their influence and impact on human health and the environment are unknown.

Most plastics are non-biodegradable; once landfilled, they may take a long and elaborate time to decompose. With an increase in the number of plastic products, especially plastic packaging, and them being discarded immediately and quickly after purchase, the amount of landfill space required for plastic waste is becoming one of the major concerns.

Cheap production and easy availability make plastic very useful. Still, it has many shortfalls, and its harmful effects are a cause of huge concern for us to save our earth and ourselves. Its few disadvantages are listed below:

• The natural decomposition of plastic can last from 400-1000 years.
• Plastics are non-biodegradable substances that take several years to decompose. Since plastic does not decompose, it is accumulating and causes environmental pollution.
• Plastic materials clog waterways, oceans, seas, lakes etc. 1 in 3 species of marine mammals have been found entangled in marine litter. 
• They cannot be burnt as when burnt, they release poisonous gases into the atmosphere, which cause air pollution.
• Plastic causes many fire hazards. 
• Many animals eat plastic materials and eventually die. Over 90% of all seabirds have plastic pieces in their stomachs. Animals like cows swallow plastic bags thrown in the garbage dump. The swallowing of such bags chokes their respiratory system and can even cause death.
• Plastic is widely used for packaging. Eating food out of plastic containers may also lead to cancer.
• Plastic bags that are carelessly thrown are responsible for the clogging of drains, leading to several inconveniences.
• The cost of recycling plastic is also very high. 
• Plastic wrappers of eatables are carelessly spilt in picnic spots, which can also lead to pollution.

Hence, we should make sure to avoid the use of plastic as much as possible.

Question 8. Explain why some fibres are called synthetic.

Answer 8: 

Some fibres are made by humans and are known as synthetic or artificial. These fibres are artificially prepared using various chemicals based on petroleum. These fibres are called synthetic fibres because they are artificially prepared using chemicals. These fibres are not made from natural fibres. These are made of small units that join together to form long chains. Some synthetic fibres are nylon, rayon, acrylic, polyester etc.

Question 9. A bucket made of plastic does not rust like a bucket of iron. Explain.

Answer 9: 

The word plastic is generally derived from the Greek word, which means capable of being shaped or moulded. Hence plastic has the property to be moulded in any desired shape. Most plastics contain organic polymers. Generally, it forms a continuous chain of carbon atoms and adds oxygen, sulphur or nitrogen. The long chain also consists of many repeat units known as monomers, and polythene is a polymer in which there are several thousand units. Rusting generally acts on the outer surface of iron, due to which the surface becomes dull. The reactive nature of iron can explain this. Iron reacts with the oxygen present in the air and forms oxides, which get corrupted. The conditions necessary for rust are mainly water and oxygen present in the environment. Plastic is a non-reactive metal, and the main condition for rust is that it should react with water and atmospheric oxygen, so plastic does not rust.

Question 10. Describe the following with examples: thermoplastics and thermosetting polymers. 

Answer 10: 

• Thermoplastic

Thermoplastics are those materials that get deformed easily on heating. Hence, they can be reused again and again. They are malleable and also can be easily bent. Some examples of thermoplastics include polyethene and PVC. Thermoplastics are mainly used in the manufacturing process and production of toys and combs.

• Thermosetting plastics

Thermosetting plastics are materials that can be moulded only once and cannot be softened by heating. A thermosetting polymer is a permanent setting polymer as it gets hardened, sets during the moulding process, and cannot be softened again. Some examples of thermosetting plastics are melamine and bakelite. Melamine can resist fire and can tolerate heat; it is used for making floor tiles, kitchenware, and fabrics. Bakelite is known to be a poor conductor of heat and electricity and is used for commercial making electrical switches, handles of various utensils, etc. 

Question 11. Name two natural fibres and sources from which they are obtained​.

Answer 11:

Examples of natural fibres include cotton, wool, flax, jute and coir. Fibres are long, thin and flexible strands and are thread-like structures. Fibres that are made by human beings are called man-made or synthetic fibres. At the same time, those fibres made from animals and plants are called natural fibres.

Source of natural fibres

The two major sources of fibres are animals and plants.

• Cotton, flax, coir, jute, rami, etc., are natural fibres from plants.
• The natural fibres of animals include alpaca, cashmere, silk, wool, etc.

Question 12. Give examples which indicate that nylon fibres are very strong.

Answer 12:

Synthetic fibres

• Synthetic fibres are man-made fibres made by chemical synthesis in contrast to natural fibres derived from living organisms.
• They are made of small units which form longer chains.

Nylon

• Nylon is synthetic fibres made up of repeating units of amides.
• Nylons are condensation polymers.
• Nylons are generally made from petroleum that can be melted to form fibres, films or shapes
• It is formed from the reaction of difunctional monomers.
• They are thermoplastic i.e can be softened through heating in nature.

Some examples below demonstrate the strength of nylon fibres.

• Nylon is used in the making of rock climbing ropes and parachutes.
• Seat belts, tyre cords and fishing nets are all nylon.
• Nylon is also used to make sporting equipment such as rackets.

Benefits of Solving Important Questions Class 8 Science Chapter

Science is an important subject that enables students to understand the functioning of the biological world and its importance. Students will see different types of theoretical and practical questions based on the activities we perform in our everyday life. Therefore, having a strong base foundation and knowledge of Science is necessary. Students are advised to understand the main concepts in the chapter. Chapter 3 Class 8 Science important questions are available on our Extramarks website. Students can quickly refer to the important questions and understand the different variety of questions which are likely to appear in the exam. 

These are the benefits of learning Important Questions Class 8 Science Chapter 3: 

• The questions are compiled from reference materials such as CBSE sample papers, NCERT books, and past years’ question papers. 
• These questions provided by Extramarks serve an important purpose in preparation for the examination. The step-wise explanatory answers are presented in easy-to-understand language.
• These important questions are an upcoming source for the Self-Assessment Module (SAM) and any upcoming exam or test.
• The questions focus on covering every aspect of the subject matter and concepts introduced to the students in the chapter with a strong emphasis on the technique of solving questions rather than just plainly providing answers alone. 
• These important questions are created based on the CBSE Curriculum Framework.
• These important questions include all the latest updates and changes in the CBSE Curriculum Framework.
• The questions mentioned in Important Questions Class 8 Science Chapter 3 are collected from various sources to ensure that they are fully authentic in content and free from errors, omissions, ambiguities, etc.
• The questions have also been written strongly emphasising the yearly examination pattern that CBSE provides for their respective class or grade level students.

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