3. Synthetic Fibres And Plastics Science class 8 in English Medium ncert book solutions Chapter Review

Chapter Review:

• Fabrics are made from fibres obtained from natural or artificial sources.
• Fibres are also used for making a large variety of household articles.
• Natural fibres like cotton,wool, silk, etc., are obtained from plants or animals. 
• The synthetic fibres, on the other hand, are made by human beings that is why these are called synthetic or man-made fibres.
• Many such small units combine to form a large single unit called a polymer.
• Polymers occur in nature also. Cotton, for example, is a polymer called cellulose.
• Cellulose is made up of a large number of glucose units.
• Such a fibre was obtained by chemical treatment of wood pulp. This fibre was called rayon or artificial silk.
• Rayon is mixed with cotton to make bed sheets or mixed with wool to make carpets.
• Nylon fibre was strong,elastic and light.It was lustrous and easy to wash.So ,it became very popular for making clothes and parachutes and ropes for rock climbing.
• PET is a very familiar form of polyester. It is used for making bottles,utensils, films, wires and many other useful products.
• Sweaters  and  shawls or blankets are prepared from another type of synthetic fibre called acrylic.
• All the synthetic fibres are prepared by a number of processes using raw materials of petroleum origin, called petrochemicals.
• Plastic can be recycled, reused, coloured, melted, rolled into sheets or made into wires.
• Polythene (Poly+ethene) is an example of a plastic. It is used for making commonly used polythene bags.
• Such plastic which gets deformed easily on heating and can be bent easily are known as thermoplastics.examples: toys, combs and various types of containers.
• There are some plastics which when moulded once, can not be softened by heating. These are called thermosetting plastics. examples: bakelite and melamine
• Plastics do not react with water and air. They are not corroded easily. That is why they are used to store various kinds of material.
• Plastic is very light, strong, durable and can be moulded into different shapes and sizes, it is used for various purposes. Plastics are generally cheaper than metals.
• Plastics are poor conductors of heat and electricity. That is why electrical wires have plastic covering, and handles of screw drivers are made of plastic.
• Teflon is a special plastic on which oil and water do not stick. It is used for nonstick coating on cookwares.
• A material which gets decomposed through natural processes, such as action by bacteria, is called biodegradable
• A material which is not easily decomposed by natural processes is termed as non-biodegradable.
• The 4 R principle is Reduce, Reuse, Recycle and Recover.