Science Class 08 part-I NCERT

1. Crop production
1. Loosened soil helps in the growth of earthworms and microbes present in the soil. Hoe is a simple tool which is used for removing weeds and for loosening the soil.
2. Fertilizer is an inorganic salt, doesn't provide any humus to the soil.
3. The best time for the removal of weeds is before they produce flowers and seeds. Weedicides are sprayed during the vegetative growth of weeds before flowering and seed formation. Spraying of weedicides may affect the health of farmers.
4. Dried neem leaves are used for storing food grains at home.
2. Microorganisms
1. Microorganisms are classified into 4 major groups. These groups are bacteria, fungi, protozoa and some algae. Viruses are also microscopic.
2. They, however, reproduce only inside the cells of the host organism, which may be a bacterium, plant or animal.
3. Microorganisms may be single-celled like bacteria, some algae and protozoa, or multicellular, such as algae and fungi.
4. They can survive under all types of environment, ranging from ice cold climate to hot springs and deserts to marshy lands.
5. Bacteria are not multicellular organisms
1. Curd contains several microorganisms. Of these, the bacterium, Lactobacillus promotes the formation of curd.
2. Yeast reproduces rapidly and produces carbon dioxide during respiration. Bubbles of the gas fill the dough and increase its volume.
3. Microorganisms are used for the large scale production of alcohol, wine and acetic acid (vinegar).
4. Yeast is used for commercial production of alcohol and wine.
6. Antibiotics are even mixed with the feed of livestock and poultry to check microbial infection in animals.
7. Some bacteria and blue green algae are able to fix nitrogen from the atmosphere to enrich soil with nitrogen and increase its fertility
8. Anthrax is a dangerous human and cattle disease caused by a bacterium. Foot and mouth disease of cattle is caused by a virus.
9. Salts and edible oils are the common chemicals generally used to check the growth of microorganisms. Therefore they are called preservatives.
1. Sodium benzoate and sodium metabisulfite are common preservatives.
2. Meat and fish are covered with dry salt to check the growth of bacteria.
3. Jams, jellies and squashes are preserved by sugar.
10. Pasteurised milk can be consumed without boiling as it is free from harmful microbes. The milk is heated to about 70 C for 15 to 30 seconds and then suddenly chilled and stored
3. Fibers
1. A synthetic fibre is also a chain of small units joined together. Each small unit is actually a chemical substance. Many such small units combine to form a large single unit called a polymer.
2. Rayon is artificial silk. Rayon is obtained from a natural source, wood pulp, yet it is a man-made fibre
3. Nylon is another man-made fibre, prepared from coal, water and air. It was the first fully synthetic fibre. Nylon thread is actually stronger than a steel wire
4. Fabric made from polyester does not get wrinkled easily. It remains crisp and is easy to wash. PET is a very familiar form of polyester used for making bottles, utensils, films, wires. Acrylic is synthetic wool
5. Synthetic fibres melt on heating. This is actually a disadvantage of synthetic fibres. If the clothes catch fire, it can be disastrous. The fabric melts and sticks to the body of the person wearing it.
6. All the synthetic fibres are prepared by a number of processes using raw materials of petroleum origin, called petrochemicals.
7. Plastic is also a polymer like the synthetic fibre. All plastics do not have the same type of arrangement of units.
8. Plastic which gets deformed easily on heating and can be bent easily are known as thermoplastics such as Polythene and PVC. Most of the thermoplastics can be recycled.
9. Plastics which when moulded once, can not be softened by heating. These are called thermosetting plastics. Example - bakelite and melamine
10. Plastics do not react with water and air. They are not corroded easily
4. Metals
1. Property of metals by which they can be beaten into thin sheets is called malleability. This is a characteristic property of metals.
2. Property of metal by which it can be drawn into wires is called ductility. Since metals produce ringing sounds, they are said to be sonorous (except some such as sodium, lead). The materials other than metals are not sonorous
3. Generally, non-metals do not react with water though they may be very reactive in air. Such non-metals are stored in water. For example, phosphorus
4. Nonmetals generally do not react with acids but metals react with acids and produce hydrogen gas.
5. If a substance cannot be broken down further by chemical reactions, by cooling, heating, or by electrolysis, it is called ‘element’. Most of the elements are metals. Less than are non-metals (oxygen, hydrogen, carbon).
5. Coal and petroleum
1. Coal is processed in industry to get some useful products such as coke, coal tar and coal gas.
1. Coke - tough, porous and black substance. It is an almost pure form of carbon. Coke is used in the manufacture of steel and in the extraction of many metals.
2. Coal tar - black, thick liquid with an unpleasant smell, starting point for materials such as naphthalene balls
3. Coal gas - obtained during the processing of coal to get coke
2. Petrol and diesel are obtained from a natural resource called petroleum. Petroleum was formed from ancient marine organisms. Oil and gas are lighter than water and do not mix with it.
3. Petroleum is a dark oily liquid. It has an unpleasant odour. It is a mixture of various constituents such as petroleum gas, petrol, diesel, lubricating oil, paraffin wax, etc.
4. Natural gas is a very important fossil fuel because it is easy to transport through pipes. Natural gas is stored under high pressure as compressed natural gas (CNG)
6. Combustion and flame
1. A chemical process in which a substance reacts with oxygen to give off heat is called combustion. The lowest temperature at which a substance catches fire is called its ignition temperature.
1. Head of the safety match contains only antimony trisulphide and potassium chlorate. The rubbing surface has powdered glass and a little red phosphorus.
2. For fires involving electrical equipment and inflammable materials like petrol, carbon dioxide (CO2 ) is the best extinguisher. CO2 being heavier than oxygen, covers the fire like a blanket
3. Substances like phosphorus burn in air at room temperature.
4. Yellow flames is because of incomplete combustion which therefore also accompanies some smoke.
5. Blue flame is due to complete combustion and smokeless and therefore produces more heat compared to yellow flame.
6. Amount of heat energy produced on complete combustion of 1 kg of a fuel is called its calorific value
7. Incomplete combustion of these fuels gives carbon monoxide gas. It is a very poisonous gas.
8. Burning of coal and diesel releases sulphur dioxide gas. It is an extremely suffocating and corrosive gas.