Hello students, in this blog you can find important summary notes from the Chapter 3, Science , NCERT Book of Class 8. These notes can be used for preparation of various school Competitive Exams, Olympiads and also developing Fundamentals.
1. Clothes are made of fabrics, and fabrics are made up of fibres that are obtained from natural or artificial sources.
2. Natural fibres like cotton, wool, silk, etc., are obtained from plants or animals & synthetic fibres, are made by human beings. and hence are called synthetic or man-made fibres.
3. Natural fibres are obtained from plants or animals, examples: like cotton, wool, silk, etc.,
4. Synthetic fibres, are made by human beings, hence these are called synthetic or man-made fibres.
5. A synthetic fibre is a chain of small units joined together and each small unit is actually a chemical substance.
6. Many small units of chemical substances combine to form a large single unit called a polymer.
7. Polymer : Made of Greek words; poly meaning many and mer meaning part/unit.
8. Polymers occur in nature: Cotton, is a polymer called cellulose.
9. Cellulose is made up of a large number of glucose units.
10. Types of Synthetic Fibres:
– Rayon is obtained from a natural source, wood pulp, yet it is a man-made/ synthetic fibre.
– Nylon is prepared from coal, water and air & it is the first fully synthetic fibre.
11. A nylon thread is actually stronger than a steel wire.
12. Polyester is suitable for dress making, because it does not get wrinkled easily, it remains crisp and is easy to wash.
13. Terylene is a popular polyester.
14. PET (polyethylene terephthalate) is a very familiar form of polyester, used for making bottles, utensils, films, wires and many other useful products.
15. Polyester (Poly + ester) is actually made up of the repeating units of a chemical called an ester.
16. Esters are the chemicals which give fruits their smell.
17. Fabrics are sold by names like polycot, polywool, terrycot, etc. These are made by mixing two types of fibres.
18. Polycot is a mixture of polyester and cotton.
19. Polywool is a mixture of polyester and wool.
20. Synthetic fibres are more durable and affordable which makes them more popular than natural fibres.
21. Synthetic fibres melt on heating, and this is actually a disadvantage of synthetic fibres.
22. All the synthetic fibres are prepared by a number of processes using raw materials of petroleum origin, called petrochemicals.
23. Plastic is a polymer like the synthetic fibre. The arrangements of units in plastics can be linear or cross-linked.
24. Plastic can be recycled, reused, coloured, melted, rolled into sheets or made into wires, hence it finds variety of uses.
25. Polythene (Poly + ethene) is an example of a plastic, used for making commonly used polythene bags.
26. Plastic which gets deformed easily on heating and can be bent easily are known as thermoplastics. Example: Polythene and PVC
27. Plastics which when moulded once, can not be softened by heating, are called thermosetting plastics. Example: bakelite and
28. Bakelite is a poor conductor of heat and electricity, hence used for making electrical switches, handles of various utensils, etc.
29. Melamine is a versatile material, it resists fire and can tolerate heat better than other plastics. It is used for making floor tiles, kitchen-ware and fabrics which resist fire.
30. plastic containers seem most convenient for storing food because of their light weight, lower price, good strength and easy handling.
31. Being lighter as compared to metals, plastics are used in cars, aircrafts and spacecrafts, too.
32. Plastics do not react with water and air and are not corroded easily, hence they are used to store various kinds of material, including many chemicals.
33. Since plastic is very light, strong, durable and can be moulded into different shapes and sizes, it is used for various purposes.
34. Plastics are generally cheaper than metals.
35. Plastics are poor conductors of heat and electricity, and therefore:
∗ electrical wires have plastic covering,
∗ handles of screw drivers are made of plastic,
∗ handles of frying pans are also made of plastic.
36. Uniforms of firemen have coating of melamine plastic to make them flame resistant.
37. A material which gets decomposed through natural processes, such as action by bacteria, is called biodegradable.
38. A material which is not easily decomposed by natural processes is termed non-biodegradable.
39. Since plastic takes several years to decompose, it is not environment friendly and major cause of environmental pollution.
40. As a responsible citizen remember the 5 R principle : Reduce, Reuse, Recycle, Recover and Refuse.