8th Science Chapter 3 - Synthetic Fibres and Plastics[Solved Questions]
Q. Name some natural fibres?
Answer: Fibres obtained from plants or animals is called natural fibres for examples cotton, wool, silk, etc.
Q. What do you mean by synthetic fibres?
Answer: Fibres made by human beings using chemical substance is called synthetic fibre. A synthetic fibre is a chain of small unit of chemical substance joined together. These small units combine to form a large single unit called a polymer. The word ‘polymer’ comes from two Greek words; poly meaning many and mer meaning part/unit. So, a polymer is made of many repeating units.
Q. Name a polymer occurs naturally?
Answer: Polymers that occur naturally is cellulose. Cellulose is made up of a large number of glucose units.
Q. Name a fibre having properties similar to that of silk.
Q. Although rayon is obtained from a natural source, wood pulp, yet it is a man-made fibre. Why?
Answer: This is because rayon is obtained by chemical treatment of wood pulp.
Q. Why does Nylon become very popular for making clothes for mankind?
Answer: Nylon is the first fully synthetic fibre prepared from coal, water and air in 1931. Nylon fibres are strong, elastic and light. It is lustrous and easy to wash. So, it became very popular for making clothes.
Q. Is nylon fibre really so strong that we can make nylon parachutes and ropes for rock climbing?
Answer: Yes, It is because nylon thread is actually stronger than a steel wire.
Q. Why does polyester fibre quite suitable for making dress material?
Answer: Fabric made from Polyester fibre does not get wrinkled easily. It remains crisp and is easy to wash. So, it is quite suitable for making dress material. Terylene is popular polyester.
Q. Name a form of polyster which is used for making bottles, utensils, films, wires and many other useful products?
Ans: PET: PET (polyethylene terephthalate) is made from two monomers, terephthalic acid and ethylene glycol, by the process called condensation polymerization
Q. Name the chemical used to make polyster?
Answer: Esters are the chemicals used to make polyster.
Q. Why synthetic fibres are more popular than natural fibres?
Answer: Synthetic fibres dry up quickly, durable, less expensive, readily available and easy to maintain which makes them more popular than natural fibres.
Q. Identify the Synthetic fibre appears to resemble wool?
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VIII Synthetic fiber and Plastic
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VIII Synthetic fiber and Plastic
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Q. What are crops?
Answer: The same kind of plants grown and cultivated at one place on a large scale is called a crop. For example, wheat, maize, etc
Q.What is the different types of crops grown in India?
Ans: These are:
(i) Kharif Crops : The crops which are sown in the rainy season (from June to September)are called Kharif crops. Paddy, maize, soya bean, groundnut, cotton, etc., are kharif crops.
(ii) Rabi Crops : The crops grown in the winter season (October to March )are called rabi crops.
Examples of rabi crops are wheat, gram, pea, mustard and linseed.
Q. Why is proper agricultural management and distribution of food necessary ?
Answer: In order to provide food for a large population—regular production, proper management and distribution of food is necessary.
Q. Why can paddy not be grown in the winter season?
Answer: Paddy requires a lot of water. Therefore, it is grown only in the rainy season.
Q. List different agricultural practice ?
Answer: The agricultural practices are listed below.
(i) Preparation of soil (ii) Sowing (iii) Adding manure and fertilizers
(iv) Irrigation (v) Protecting from weeds (vi) Harvesting (vii) Storage
Q. Why does the loosening of soil allow the roots to breathe easily?
Answer: The process of loosening and turning of the soil is called tilling or ploughing.This is done by using a plough.
The air enters easily through loosen soil and the root can breathe easily even when they go deep into the soil.
Q. Why earthworms are called farmers friends?
Ans: The loosened soil helps in the growth of earthworms and microbes present in the soil. These organisms are friends of the farmer since they further turn and loosen the soil and add humus to it.
Q.why does the soil need to be turned and loosened before seeds are sown?
Answer: Since only a few centimeters of the top layer of soil supports plant growth, turning and loosening of soil brings the nutrient-rich soil to the top so that plants can use these nutrients.
Q. What are the purposes of the leveling of soil?
Answer: The ploughed field may have big pieces of soil called crumbs. It is necessary to break these crumbs with a plank. The field is levelled for sowing as well as for irrigation purposes. The levelling of soil is done with the help of a leveller.
Q.Why we sometimes add manure to the soil before tilling?
Answer: This helps in proper mixing of manure with soil.
Q. Name some agricultural Implements?
Answer: The main tools used for agriculture purpose are the plough, hoe and cultivator.
Q.How can we separate good, healthy seeds from the damaged ones?
Answer: We put seeds into bucket full of water. Damaged seeds being hollow became lighter therefore they float on water and remaining good seeds sink at bottom.
Q. Why sowing seeds with seed drill is better than by hand?
Answer: Practice of sowing seeds by hand is called broadcasting. The seed drill is used for sowing with the help of tractors. This sows the seeds uniformly at proper distances and depths. It ensures that seeds get covered by the soil after sowing. This also prevents damage caused by birds.
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(i) The rational number that does not have a reciprocal.
(ii) The rational numbers that are equal to their reciprocals.
(iii) The rational number that is equal to its negative.
Answer: (i) 0 (ii) 1 and (–1) (iii) 0
Q. 2. Fill in the blanks.
(i) Zero has ________ reciprocal.
(ii) The numbers ________ and ________ are their own reciprocals
(iii) The reciprocal of – 5 is ________.
(iv) Reciprocal of 1/x , where x ≠ 0 is ________.
(v) The product of two rational numbers is always a _______.
(vi) The reciprocal of a positive rational number is ________.
Answer: 11. (i) No (ii) 1, –1 (iii) -1/5 (iv) x (v) Rational number (vi) positive
Q.3. Write any 3 rational numbers between –2 and 0.
Q.4. Find any ten rational numbers between -5/6 and 5/8
Q.5. Represent these numbers on the number line. (i) 7/4 (ii) -5/6(iii) 36/7(iv) 21/3
Q.6. Multiple Choice Questions
1. A rational number p/q is said to be in the simplest form if the HCF of p and q is
(a) 2 (b) 1 (c) 0 (d) 3 [A]
2. Between any two distinct rational numbers there exist
(a) Finite rational numbers (b) Infinite rational numbers (c) No rational number (d) none of the above [B]
3. A rational number a/b is greater than c/d if
(a) ad > bc (b) ad < bc (c) ad = bc (d) ad ≠ bc [A]
4. Is zero a rational number
(a) Yes (b) No (c) Can’t say [A]
5. Rational numbers are not closed under
(a) Addition (b) Multiplication (c) Division (d) Subtraction [C]
7. What number should be added to −78to get 49?
8. The sum of two rational numbers is −12. If one of the numbers is 56, find the other.
9. What number should be subtracted from −23 to get −12?
10. Divide the sum of 135and−127by the product of −317and1−2.
11. The product of two rational numbers is −169. If one of the numbers is −43, find the other.
12. A drum full of rice weights 4016kg. If the empty drum weights 1334kg, find the weight of rice in the drum.
13. A car is moving at an average speed of 4025km/h. How much distance will it cover in 712h?
14. Raju earns Rs16000/m. He spends 14of his income on food; 310of the remainder on house rent and 521of the remainder on education of children. How much money is still left with him?
15. After reading 79of a book, 40pages are left. How many pages are there in the book?
16. If you subtract 1/2 from a number and multiply the result by 1/2, you get 1/8. What is the number?
17. Three consecutive integers are such that when they are taken in increasing order and multiplied by 2, 3, and 4 respectively, they add up to 74. Find these numbers.
Further Study Visit links given below:
Class VIII Mathematics
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Physics Chemistry Biology 8th Sample paper 8th S. Science
Q. Why we need food?
Ans: Food is a basic necessity for all living organisms to survive. It is because food provides energy to all living organisms to do their life activities. Food also helps them to grow and build their bodies
Q. What is nutrition?
Ans: The mode of taking food by an organism and utilizing it by the body is called nutrition.
Q. What are the modes of nutrition?
Ans: There are two modes of nutrition in organisms. They are autotrophic and heterotrophic nutrition
The mode of nutrition in which organisms make their own food is called Autotrophic Nutrition and eg. Green plants.
The mode of nutrition in which organisms depend on others for their food, is called Heterotrophic nutrition. E.g. All animals, including human beings.
Q. Why the sun is called the ultimate source of energy for all forms of life?
Ans: In the absence of the sun, there would be no photosynthesis. Hence, there would not be any food. In the absence of food, life would be impossible on earth. So, the sun is the ultimate source of energy for all forms of life.
For full Study visit:
CBSE NCERT Class 7 Science Nutrition in Plants
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