Hello students, in this blog you can find important summary notes from the Chapter 6, Science , NCERT Book of Class 8. These notes can be used for preparation of various school Competitive Exams, Olympiads and also developing strong Fundamentals.
1. Magnesium ribbon burns to form magnesium oxide and produces heat and light.
2. Coal burns in air producing carbon dioxide, heat and light.
3. Combustion : A chemical process in which a substance reacts with oxygen to give off heat.
4. Fuel : The substance that undergoes combustion is said to be combustible, also called a fuel.
5. Food is a fuel for our body, because food is broken down by reaction with oxygen and heat is produced.
6. Air is necessary for combustion.
7. Ignition temperature : It is the lowest temperature at which a substance catches fire.
8. The head of the safety match contains : antimony trisulphide and potassium chlorate.
9. The rubbing surface of match box has powdered glass and a little red phosphorus.
10. When the match is struck against the rubbing surface, some red phosphorus gets converted into white phosphorus. This immediately reacts with potassium chlorate in the matchstick head to produce enough
heat to ignite antimony trisulphide and start the combustion.
11. Ignition temperature of kerosene oil is lower than that of wood.
12. We can boil water in the paper cup, because the heat supplied to the paper cup is transferred to water by conduction. So, in the presence of water, the ignition temperature of paper is not reached. Hence, it does not burn.
13. Inflammable substances : These are the substances which have very low ignition temperature and can easily catch fire with a flame.
Examples : petrol, alcohol, Liquified Petroleum Gas (LPG) etc.
14. Fire is extinguished by a fire brigade, because : – When a fire brigade arrives, it pours water on the fire. – Water cools the combustible material so that its temperature is brought below its ignition temperature. This prevents the
fire from spreading.
– Water vapours also surround the combustible material, helping
in cutting off the supply of air.
15. 3 essential requirements for producing fire:
– air (to supply oxygen)
– heat (to raise the temperature of the fuel beyond the ignition temperature)
16. Rapid Combustion : When the fuel burns rapidly and produces heat and light. Such combustion is known as rapid combustion.
Example : Burning matchstick or a gas lighter near a gas stove turned on, in the kitchen.
17. Phosphorus burns in air at room temperature.
18. Spontaneous Combustion : The type of combustion in which a material suddenly bursts into flames, without the application of any apparent cause is called spontaneous combustion.
Examples : burning of coal dust, forest fires due to heat of sun or lightning strikes.
19. Explosion : When a cracker is ignited, a sudden reaction takes place with the evolution of heat, light and sound. A large amount of gas formed in the reaction is liberated. Such a reaction is called explosion.
20 The substances which vapourise during burning, give flames.
Example, kerosene oil and molten wax
21. Charcoal, does not vapourise and so does not produce a flame.
22. Goldsmiths blow the outermost zone of a flame with a metallic blow-pipe for melting gold and silver, because the outermost zone of the flame is the hottest.
23. Fuel : The sources of heat energy for domestic and industrial purposes like wood, charcoal, petrol, kerosene etc, are called fuels.
24. Features of a good fuel :
– is readily available
– is cheap
– produces a large amount of heat – does not leave behind any undersirable substances.
25. There is probably no fuel that could be considered as an ideal fuel.
26. Calorific value : The amount of heat energy produced on complete combustion of 1 kg of a fuel is called its calorific value.
27. Unit of calorific value : expressed in a unit called kilojoule per kg (kJ/kg).
28. Wood burning has been replaced by coal and other fuels like LPG, because :
– burning of wood gives a lot of smoke which is very harmful for human beings
– it causes respiratory problem
– trees provide us with useful substances which are lost when wood is used as fuel
– cutting of trees leads to deforestation which is quite harmful to the environment
29. The increasing fuel consumption has harmful effects on the environment in following ways :
– Carbon fuels like wood, coal, petroleum release unburnt carbon particles which are dangerous pollutants causing respiratory
– Incomplete combustion of these fuels gives carbon monoxide gas which is a very poisonous gas.
– Combustion of most fuels releases carbon dioxide in the environment causing global warming.
– Burning of coal and diesel releases sulphur dioxide gas, which is an extremely suffocating and corrosive gas.
30. It is dangerous to burn coal in a closed room, because the carbon monoxide gas produced can kill persons sleeping in that room.
31. Global warming : it is the rise in temperature of the atmosphere of the earth.
32. Global warming, results in the melting of polar glaciers, which leads to a rise in the sea level, causing floods in the coastal areas.
33. Acid rain : Oxides of sulphur and nitrogen dissolve in rain water and form acids. Such a rain is called acid rain.