Chapter 6 Flames and Combustion Class 8 Notes | EduRev

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Class 8 : Chapter 6 Flames and Combustion Class 8 Notes | EduRev

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Chapter 6 Combustion and flames 
Combustion – A chemical reaction of a substance to give out heat and light is called combustion. 
Fuel – The substance that undergoes combustion is known as fuel. 
Ignition temperature – the lowest temperature at which a substance catches fire is called its ignition temperature. 
Calorific value – the amount of heat energy produced on burning one kg of fuel is called its calorific value. The calorific value of petrol 
is higher than wood because 1kg of petrol produces more energy than the same amount of wood. 
Inflammable substances - the substances that easily catches fire because they have a low ignition temperature are known as 
inflammable substances. E.g. petrol 
Conditions necessary for fire: 
a. Oxygen 
b. Fuel 
c. Heat to raise the temperature of a substance to its ignition temperature. 
If any of the above factor is taken away, the fire will extinguish. For e.g. a fire extinguisher releases carbon dioxide that forms a layer 
on the burning material and prevents its exposure to the air (and cut off the supply of oxygen), which eventually extinguishes the fire. 
Water reduces the temperature of the burning material and bring it lower than the ignition temperature. 
Types of Combustion 
a. Rapid combustion – A type of combustion, which occurs rapidly. E.g. Burning of LPG or petrol. 
b. Spontaneous combustion – A type of combustion in which a material suddenly catches fire without any external source of 
heat. E.g. burning of sodium and phosphorus at room temperature. Forest fires and burning of coal dust in hot summers are 
also the examples of spontaneous combustion. 
c. Explosion – a type of combustion, which is accompanied by a blast, and sudden releases of excessive amount of heat and light 
is known as explosion. E.g. fireworks and blasts. 
Flame 
It is formed due to the vaporized material while burning of a substance. For e.g. If you hold a glass slide on the top of a candle the 
vaporizing carbon will be collected on it forming a black patch. 
  
 
 
 
 
 
 
 
 
 
Harmful effects of fuels: 
1. The burning of fossil fuels produces harmful gases such as carbon dioxide, carbon monoxide, oxides of sulfur and nitrogen that 
can cause various respiratory disease such as asthma. 
2. Increased levels of carbon dioxide causes global warming. The rise in average temperature of earth due to the trapped sunrays 
in the Earth’s atmosphere is known as global warming.  
3. Oxides of nitrogen dissolve in rainwater and form acid. Such type of rain is known as acid rain and harm crops, soils and cause 
skin burn. 
Alternative sources of energy such as Compressed Natural Gas, Liquefied Petroleum Gas, solar energy and wind energy should be 
used to reduce the release of such harmful gases. 
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