NCERT Solution Class 8 Science – Chapter 3 Synthetic Fibres and Plastics

There are some fibers that are prepared by man by using chemicals. These are called synthetic fibers. These are made of small units that join together to form long chains. Examples of synthetic fibers are rayon, nylon, polyester, acrylic, etc.

Question 2: Mark (tick) the correct answer.

Rayon is different from synthetic fibers because

(a) it has a silk-like appearance.

(b) it is obtained from wood pulp.

(c) its fibers can also be woven like those of natural fibers.

Rayon is different from synthetic fibers because

(a) It has a silk-like appearance.

(b) It is obtained from wood pulp. (Correct)

(c) Its fibers can also be woven like those of natural fibers.

Question 3: Fill in the blanks with appropriate words.

(a) Synthetic fibers are also called__or__fibers.

(b) Synthetic fibers are synthesized from raw materials called__.

(c) Like synthetic fibers, plastic is also a__.

(a) Synthetic fibers are also called artificial or man-made fibers.

(b) Synthetic fibers are synthesized from raw materials called petrochemicals.

(c) Like synthetic fibers, plastic is also a polymer.

Nylon fibers are very strong. It is used for making ropes used for climbing rocks and for making parachutes. Their usage shows that nylon fibers have high tensile strength.

The characteristics that make plastics favorable for storing food items are:

a)    Light weight

b)    Lower price

c)    Good strength

d)    Easy handling

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

There are two types of plastics: Thermosetting plastics and Thermoplastics.

Thermosetting plastic cannot be softened by heating. Thus, it cannot be reshaped once moulded.

Thermoplastic can be softened easily by heating. Thus, it can be reshaped.

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

a) Saucepan handles

b) Electric plugs/switches/plug boards

a) Saucepan handles are made of thermosetting plastics because these plastics do not get softened on heating. Also, thermosetting plastics such as Bakelite are poor conductors of heat.

b) Thermosetting plastics such as Bakelite are poor conductors of heat and electricity. Therefore, they are used for making electric plugs, switches, plug boards, etc.

Question 8: Categorize the materials of the following products into ‘can be recycled’ and ‘cannot be recycled’.

Telephone instruments, plastic toys, cooker handles, carry bags, ball point pens, plastic bowls, and plastic covering on electrical wires, plastic chairs and electrical switches.

Rana should buy shirts made from cotton. This is because cotton is a good absorber of water. It can soak the sweat coming out of our body and expose it to the environment. Thus, it helps in evaporating the liquid (sweat), thereby cooling our body.

Plastics are not corroded even if they come in contact with strong chemicals. This is because of their non-reactive nature with most materials. For example, the cleansing chemicals that we use at home are stored in plastic bottles, instead of metal containers.

Question 11: Should the handle and bristles of a tooth brush be made of the same material?

Explain your answer.

No. The handle and bristles of a tooth brush should be made of different materials. The handle of a toothbrush should be hard and strong, while the bristles should be soft and flexible.

Plastics are non-biodegradable. Once introduced into the environment, they take several years to decompose. Plastics add to the environmental pollution. They cannot be burnt as when burnt, they release poisonous gases. Plastic bags thrown in the garbage dump are swallowed by animals like cows. These plastic bags choke their respiratory system and can even prove fatal. Therefore, we should avoid plastics as far as possible.

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

Question 14: ‘Manufacturing synthetic fibers is actually helping conservation of forests’. Comment.

Raw materials for natural fibers are mainly derived from plants and this means cutting a lot of trees. This leads to deforestation. But raw materials of synthetic materials are mainly petrochemicals. Hence, manufacturing synthetic fibers helps in the conservation of forests.

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

We will design a circuit to see that thermoplastics are poor conductors of electricity.

We need a bulb, some wires, a battery, a piece of metal, and a plastic pipe. Set up the circuit first with the metal and then with the plastic pipe (as shown in the figure).

After you switch on the current, you will observe that the bulb glows in the former case. In the latter case, the bulb does not glow. Hence, a plastic pipe (which is a thermoplastic) is shown to be a poor conductor of electricity.