Solved examples for class 8 science : Chapter 3
Synthetic Fibres and Plastics
I. Multiple Choice Questions (MCQs).
Q.1. The first synthetic fibre made completely from chemicals, derived from coal along with water and air, is
d) none of these
Answer c) nylon
Q.2. The synthetic fibre which resembles silk fibres in appearance, is
Answer. (d) rayon
Q.3. Terylene, dacron and terene are some of the
a) nylon fibres
b) polyester fibres
c) rayon fibres
d) none of these
Answer: (b) polyester fibres
Q.4. A synthetic fibre which has properties similar to wool, is
Answer: (d) acrylic
Q.5. Which of the following is an example of thermosetting plastics?
Answer. d) Bakelite
II. Fill in the blanks by selecting appropriate words from the given list :
List : cellulose, Thermoplastics, Thermosetting plastics, Blended, polymer
Q.1. Synthetic fibres are made of long chains of repeating units called……………………………….
Q.2. Rayon is prepared from…………………………………… obtained from wood pulp.
Q.3. ………………………….. fibre is generally prepared by mixing a synthetic fibre to a natural fibre.
Q.4. ……………………….. bend easily on applying pressure and get deformed on heating.
Q.5. . ………………………….. are hard and stiff and break on applying pressure.
Answer: 1- polymer, 2- cellulose, 3- Blended ,4- Thermoplastics, 5- Thermosetting plastics
III. Match the items of Column A with those of Column B.
Answer: 1-e, 2-a, 3-d, 4-b, 5-c,
IV. Write ‘T’ for the true and ‘F’ for the false statements.
Q.1. Cotton is made up of protein.
Q.2. Terylene is a very popular polyester fibre which has properties just like rayon.
Q.3. Clothes made from blended fibres are more comfortable to wear since they allow air to pass and retain moisture.
Q.4. The sophisticated waterproof acrylic fabric retains its colour for years even though it is exposed to the sun and the rain.
Q.5. Burning of plastic is a fast process which releases a lot of useful gases in the atmosphere.
Answer: 1-F, 2-F, 3-T, 4-T, 5-F
V. Very Short Answer Questions.
Q.1. What is the monomer of polythene?
Answer: Ethene is the monomer of polythene.
Q.2. Name a synthetic fibre which resembles wool.
Answer: Acrylic is a synthetic fibre which has properties similar to wool.
Q.3. Which property of perspex is used for making lenses?
Answer: Transparent property of perspex is used for making lenses.
Q.4. Name a type of plastic which is flame resistant.
Answer: Melamine plastic is flame resistant.
Q.5. Why is polystyrene used as an insulating material in refrigerators?
Answer: Thermoplastic polystyrene used as an insulating material in refrigerators.
VI. Short Answer Questions.
Q.1. Which type of plastic cannot be remoulded and why?
Answer: Thermosetting plastics can be moulded into shape only once and cannot be softened or melted on reheating. This is because the chains get highly cross linked on heating. Bakelite and melamine are some examples of thermosetting plastics.
Q.2. Why is recycled plastic not used to store foodstuff?
Answer: Colouring agents added during recycling of plastics which makes it unsafe to store food items.
Q.3. Why is incineration of plastics a better method of disposal of plastics?
Answer: Incineration of plastics is a better method of disposal of plastics because plastics at very high temperatures reduces the volume of wastes, provides useful supply of heat energy and also breakdown the polluting gases produced during the reaction.
Q.4. What do you mean by synthetic fibre? Describe its monomer and polymer briefly.
Answer: Synthetic fibres are man-made from chemicals. They are generally based on polymers and are stronger than natural and regenerated fibres.
Synthetic fibres are made of long chains of repeating units called polymer. The word polymer is derived from the Greek words poly [meaning many] and mer [meaning unit or part]. Each of the smaller units in the polymer is known as monomer. For example, ethene is the monomer of polyethene or polythene.
Q.5. What is rayon fibre? How is it prepared? Mention its uses also.
Answer: Rayon was the first synthetic fibre to be invented at the end of the nineteenth century.
It resembles silk fibres in appearance and so it is known as artificial silk. Rayon is prepared from cellulose obtained from wood pulp.
Uses of Rayon
1. Rayon is used as a fabric for clothing since it is soft, comfortable and absorbent like cotton and lustrous like silk.
2. It is used in the manufacturing of carpets, when blended with wool.
3. It is used to make automobile tyre chords since it is a strong fibre.
4. It is also used to make bandages and surgical dressing for covering wounds, due to its highly absorbing nature.
Q.6. What do you understand by nylon fibre? Mention its properties and uses briefly.
Answer: Nylon was the first synthetic fibre made completely from chemicals, derived from coal along with water and air.
Properties of nylon
The fibre is strong, light, lustrous, elastic and wrinkle free. It is easy to wash as it does not absorb water. Fungus or moth do not attack the fibre. All these properties make it a very popular clothing material.
Uses of Nylon
• Nylon fibre is used for making ropes, tyre chords, small machine parts, fishing nets and parachute fibres since it is strong and light weight. The fibre is even stronger than a steel wire of comparable size. Therefore, the ropes made from it can be used for mountaineering.
• It is used for making socks, stockings and combat uniforms because of its strength and elastic nature.
• It is also used to make garments and swimming wear as it dries quickly, is light and wrinkle
• Toothbrushes, combs, etc., are other articles made from nylon.
VII. Long Answer Questions.
Q.1. What do you understand by polyester and acrylic fibre? How and why is polyester blended with other fibres? Describe properties and uses of polyester fibre.
Answer: Polyester is synthetic fibre made from petroleum. Terylene, dacron and terene are some polyester fibres.
Acrylic is synthetic fibre which has properties similar to wool. It is cheaper than wool and can be dyed into a variety of colours.
Polyester is made up of the monomer, ester. The ester units are repeated to produce polyester [poly+ ester]. Esters are organic substances which give fruits like their characteristic smell. Polyester is blended with cotton to produce polycot and with wool to produce polywool.
Properties of polyester fibres
Polyester fibres are strong, do not wrinkle, retain their crease, absorb very little water and dry quickly. Like nylon they can also be drawn into very thin fibres which are woven into clothes.
Uses of Polyester
• Polyester is used for making sarees, dress materials, bed sheets, curtains, etc., because it does not stretch, shrink or wrinkle. The products are durable and can be worn or washed frequently.
• Fire hoses and conveyer belts are made from polyester since they are lightweight and compact.
• It is used to make lightweight sails which are resistant to sea salts.
• It is used for making jackets, other water resistant garments and carpets.
• It is used in the industry to make filters, ropes and films.
• PET bottles and jars made from it are used for storing grains and other food items.
Q.2. (a) Mention some characteristics of synthetic fibres.
(b) How is pollution caused due to excessive use of plastics?
Answer a) Characteristics of Synthetic Fibres
• Synthetic fibres are strong, light, durable, easy to wash and moth proof. They do not shrink or wrinkle and therefore need little or no ironing.
• Since they do not absorb water or moisture, they dry quickly. The fabrics stick to the body and make the wearer uncomfortable during summer.
• Synthetic fibres catch fire easily. They burn, shrink and melt forming beads which stick to the skin.
• These fibres are non-biodegradable and cause soil pollution. The fabrics made of synthetic fibres, develop a charge known as static electricity in the dry weather. This causes skin problems in many people.
b) Pollution due to excessive use of plastics
Plastics are non-biodegradable materials like plastic bags take several years to decompose.
• The careless and irresponsible way of disposal of plastics chokes the drains and blocks the soil, Thereby reducing the percolation of water. The plants growing in those areas do not get enough water from the soil.
• Plastics bags swallowed by cows while feeding on garbage choke their respiratory system or form a lining in the stomach which ultimately leads to their death.
• The dyes used in recycled plastic materials contaminate food stuff which if consumed can lead to cancer.
• Since they are light weight, they can be blown about and get caught in trees and collect in open drains.
• The toxic substances present in plastic wastes dumped in rivers cause the death of fishes and other aquatic animals.
• Burning of plastic is a slow process which releases a lot of poisonous gases into the atmosphere.
Q.3. (a) What do you understand by plastics? Mention its kinds and characteristics briefly. Write some commonly used plastics and their uses.
(b) Describe disposal of plastic waste and conservation action.
Answer a) Plastics are polymers in which the monomers are arranged in a linear manner or they may be cross linked.
Plastics are of two kinds:
(i) Thermoplastics :
(ii) Thermosetting plastics :
Characteristics of Plastics
• Plastics are light, strong, durable and easy to handle.
• They can be moulded into any desired shape and size and can also be coloured. Most of the plastics are transparent or translucent.
• They are poor conductors of heat and electricity.
• They do not corrode or rust or react with chemicals.
• Plastics can be made on a large scale and some types of plastic can be recycled. But colouring agents added during recycling makes it unsafe to store food items.
• Plastics are waterproof and air tight and are used to store several solid and liquid food items like chips, biscuits, milk, oil, etc.
Some commonly used plastics and their uses
b) The following steps should be taken for plastic disposal and conservation of the environment :
• Segregating biodegradable and non-biodegradable wastes and disposing them accordingly.
• Disposing plastic in proper places and not in the sewage system, on roads, streets
or on barren Lands.
• Reducing the amount of plastic used with recycled paper bags, jute or cotton bags, metal containers, etc.
• Recycling or reusing plastics. The plastic is collected, sorted, melted and remoulded.
• Incineration or burning of plastics at very high temperatures reduces the volume of wastes, provides useful supply of heat energy and also breakdown the polluting gases produced during the reaction.
Krishna Kant Majee