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

Synthetic Fibres and Plastics

Question 1.
Explain why some fibres are called synthetic?

Answer:
Since man-made fibres are synthesised from petrochemicals, they are called synthetic fibres.

Question 2.
Mark (✓) the correct answer.

Rayon is different from synthetic fibres because
(a) It has a silk-like appearance.
(b) It is obtained from wood pulp.
(c) Its fibres can be woven like those of natural fibres.

Answer:
(b) It is obtained from wood pulp.

Question 3.
Fill in the blank spaces with appropriate words.

(a) Synthetic fibres are also called ________ or ________ fibres.
Answer:

Artificial OR Man-made

(b) Synthetic fibres are synthesised from raw material called ________.
Answer:
Petrochemicals

(c) Like synthetic fibres, plastic is also a ________
Answer:
Polymer

Question 4.
Give examples which indicate that nylon fibres are very strong.

Answer:
They are used to make parachutes, and ropes for rock climbing.

Question 5.
Explain why plastic containers are favoured for storing food.

Answer:
The three main advantages of using plastic containers for storing food are :

  1. They have lightweight
  2. Good strength
  3. Are easy to handle

Question 6.
Explain the difference between the thermoplastic and thermosetting plastics.

Answer:

ThermoplasticsThermosetting plastics
(i) These are the plastics which gets deformed easily on heating and can be bent easily.Example: Polythene and P.V.C.These are the plastics which when
moulded once, cannot be softened by heating. Example: Bakelite and melamine.
(ii) These are used for manufacturing toys, combs, car grills and various types of containers.(a) Bakelite is used for making electrical switches, handles of various utensils, etc.
(b) Melamine is used for making floor tiles, kitchen ­wares and fabrics, which resist fire.

Question 7.
Explain why the following are made of thermosetting plastics :

(a) Saucepan handles

Answer:
As bakelite (a kind of thermosetting plastic) is a bad conductor of heat, therefore, saucepan handles are made up of it.

(b) Electric plug/switches/plugboards

Answer:
Bakelite (a kind of thermosetting plastic) is also a bad conductor of electricity, so electric plugs, switches and plugboards are made up of it.

Question 8.
Categorise the materials of the following products into “can be recycled” and “cannot be recycled” :
Telephone instruments, plastic toys, cooker handles, carry bags, ballpoint pens, plastic bowls, plastic- covering on electric wires, plastic chairs, electrical switches.

Answer:
(1) Can be recycled: Plastic toys, ballpoint pens, plastic bowls, carry bags, plastic covering on electric wires. (As these are thermoplastics, so these can be recycled).

(2) Cannot be recycled: Telephone instruments, cooker handles, plastic chairs, etc. (As these are thermosetting plastics, these cannot be recycled).

Question 9.
Rana wants to buy shirts for summer. Should he buy cotton shirts or shirts made from synthetic material? Advise Rana, giving your reason.

Answer:
I would advise Rana to buy cotton shirts as cotton shirts absorb the sweat and thus endorse cooling. Besides, they also provide aeration.

Question 10.
Give examples to show that plastics are noncorrosive in nature.

Answer:
The plastics are noncorrosive, that is, they do not react or do not have a chemical reaction with the materials contained in it. That is why many containers like a bucket, mug, water bottles, food containers are made by using plastics.

Question 11.
Should the handles and bristles of a toothbrush be made of the same material? Explain your answer.

Answer:
The handles and bristles of a toothbrush should be made of a material which has lightweight good strength and is hygiene. “But the bristles should be soft enough so that ” they do not harm gums while the handle should be quite stiff so that it may not get bent while brushing.

Question 12.
‘Avoid plastics as far as possible ’. Comment on this advice.

Answer:
Since plastic takes several years to decompose, it is not environment friendly. It causes environmental pollution. Besides, its burning releases lots of poisonous fumes into the atmosphere causing air pollution. So, we should avoid the use of plastics as far as possible.

Question 13.
Match the terms of column A correctly with the phrases given in column B :

AB
PolyesterPrepared by using wood pulp
TeflonUsed for making parachutes and stockings
RayonUsed to make non­ stick cookware
NylonFabrics do not wrinkle easily

Answer:

AB
PolyesterFabrics do not wrinkle easily
TeflonUsed to make non­ stick cookware
RayonPrepared by using wood pulp
NylonUsed for making parachutes and stockings

Question 14.
“Manufacturing synthetic fibres are actually helping conservation of forests.” Comment.

Answer:
The synthetic fibres are made up of petrochemicals. So, manufacturing synthetic fibres does not depend upon plants. These synthetic fibres cater to the need of people up to great extent.

Thus, the forests are not destroyed to manufacture clothes and other items. So, indirectly, we can come to the conclusion that manufacturing synthetic fibres is actually helping the conservation of forests.

Question 15.
Describe an activity to show that thermoplastic is a poor conductor of electricity.

Answer:
If we make an experimental set up using copper wire, a thermoplastic object, a bulb and a cell, as shown in the figure given below, the bulb does not glow.
NCERT Solutions for Class 8 Science img 4
However, if the thermoplastic object is removed from the circuit, the bulb glows up. This shows that thermoplastic objects are bad conductor of electricity.

InText Questions and Answers

Question 1.
Can you name some natural fibres?

Answer:
Yes.

  • Cotton
  • Wool
  • Silk, etc., are natural fibres.

Question 2.
Make a list of some common articles made from fibres. Try to separate them into those made from natural fibres and those made from artificial fibres. Make entries in Table 3.1 (Textbook, P. 32).

Answer:

S.NoName of ArticleType of Fibre (Natural! artificial)
1.Cotton clothesNatural
2.Woollen sweaterNatural
3.Nylon sareeArtificial
4.Woollen socksNatural
5.Polyester shirtArtificial

Question 3.
Why did you label some fabrics as artificial in the above table?

Answer:
They are not natural fibres but are man-made, i.e., artificially made. That is why man-made fabrics are labelled as artificial.

3.1 What are Synthetic Fibres?

Question 4.
Try to recall the uniform pattern found in a necklace of beads joined with the help of a thread Fig. 3.1 (a) (given on Textbook, P. 32). Or, try to join a number of paper clips together to make a long chain, as in Fig. 3.1 (6). Is there any similarity between the two?

Answer:
Yes, both are a chain of small units joined together.

3.2 Types of Synthetic Fibres

Question 5.
Is nylon fibre really so strong that we can make nylon parachutes and ropes for rock-climbing?

Answer:
Yes, nylon fibres are very strong, elastic and light. It is scientifically tested that a nylon wire is stronger than a steel wire. That is why nylon fibres are used for making parachutes and ropes for rock climbing.

Activity 3.1

Question 1.
Tabulate the data as shown in Table 3.2 (Textbook, P. 34).

Answer:

S.NoType of Thread FibreTotal weight required to break the Thread
1.Cotton30 g
2.Wool60 g
3.Silk70 g
4.Nylon100 g

Question 6.
My mother always buys PET water bottles and PET jars for storing rice and sugar. I wonder, what PET is!

Answer:
PET is a very familiar form of polyester. It is used for making bottles, utensils, films, wires and many other useful products.

3.3 Characteristics of Synthetic Fibres

Question 7.
Imagine that it is a rainy day. What kind of umbrella would you use and why?

Answer:
I will use an umbrella made up of synthetic cloth because it dries up soon, is durable, less expensive, readily available and easy to maintain.

Question 8.
Do synthetic fabrics soak less/ more water than the natural fabrics? Do they take less/more time to dry?

Answer:
Yes, synthetic fabrics soak less water than the natural fabrics and they take less time to dry.

Question 9.
What does the activity (Activity 3.2) tell you about the characteristics of the synthetic fabrics?

Answer:
Synthetic fibres soak less water and take less time to dry as compared to the natural fibres.

Question 10.
Find out from your parents about the durability, cost and maintenance of synthetic fabrics, compared with natural fabrics.

Answer:
Synthetic fibres are more durable, cheaper and need less maintenance than natural fabrics.

3.4 Plastics

Question 11.
Try to bend some plastic articles. Can all the plastic articles be bent easily?

Answer:
No, all the plastic articles cannot be bent easily. Some plastic articles can bend easily while some break when forced to bend

3.5 Plastics as Materials of Choice

Question 12.
What is the material of the buckets or mug you are using now? What are the advantages of using a plastic container?

Answer:
We use buckets or mugs made up of plastics. Plastic is a very light, strong, durable material and can be moulded into different shapes and sizes. So it is used for various purposes.

Question 13.
Make a list of different kinds of plastic containers that you use in daily life.

Answer:

  1. Bucket
  2. Mug
  3. Dustbins
  4. Water tank
  5. Carry bag

3.6 Plastic and the Environment

Question 14.
Disposal of plastic is a major problem. Why?

Answer:
This is because plastics do not decompose easily through natural processes, such as the action of bacteria, etc. Thus, they cause environmental pollution.

Question 15.
Can you suggest some other ways in which you can contribute towards reducing the use of plastic materials?

Answer:
Plastic jars, bottles and other containers should not be thrown away only after using once. They can be used again to store pulses, spices, etc. Besides, the use of plastic tiffins, glasses, spoons, etc. should be avoided. Utensils made of steel can be used. In fact, avoiding the use of plastic as far as possible is the best in reducing the use of plastic materials.

Take Your IIT JEE Coaching to Next Level with Examtube

  • Mentoring & Teaching by IITians
  • Regular Testing & Analysis
  • Preparation for Various Engineering Entrance Exams
  • Support for School/Board Exams
  • 24/7 Doubts Clarification