Class 8
chapter 6
subject science
combustion and flame
combustion :- a chemical process in which a substance react with oxygen to give of heat is called combustion.
* The substances that undergoes combustion is said to be combustible. it is also called a fuel.
* The fuel maybe solid, liquid or gas.
* Oxygen (in air) is essential for combustion.
* During the process of combustion heat and light are given out.
* Ignition temperature is the lowest temperature at which a combustible substance catches fire.
* In the reactions mentioned above magnesium and charcoal are combustible substances.
Ignition temperature :- the lowest temperature at which a substance catch fire is called its ignition temperature.
* Combustible substance cannot catch fire or burn as long as its temperature is lower than its admission temperature.
* Kerosene oil and wood do not catch fire on their own at room temperature.
* If kerosene oil is heated a little, it will catch fire.
* But if wood is heated a little it would still not catch fire.
* Ignition temperature of kerosene oil is lower than that of wood.
Inflammable substances :- the substances which have very low admission temperature and can easily catch fire with a flame are called inflammable substances.
Ex. Petrol, LPG, alcohol, kerosene.
There are three essential requirements for producing fire these are- fuel, air (to supply Oxygen) and heat ( to raise the temperature of the fuel beyond the ignition temperature)
Fire can be controlled by removing one or more of these requirements.
The job of a fire extinguisher is cut off the supply of air or to bring down the temperature of the fuel or both.
* The most common fire extinguisher is water. But water works only when things like wood and paper are on fire.
* Water is not suitable for fires involving electrical equipment, oil and petrol.
For fire involving electrical equipment and inflammable materials like petrol, carbon dioxide is the best extinguisher. carbon dioxide being heavier than oxygen, covers the fire like a blanket. Since the contact between the fuel and oxygen is cut off, the fire is controlled. The added advantage of carbon dioxide is that in most cases it does not harm the electrical equipment.
Types of combustion :-
1. Rapid combustion :- gas burns rapidly and produce heat and light. Sach combustion is known as rapid combustion.
2. Spontaneous combustion :- the type of combustion in which a material suddenly bursts into flame without the application of any apparent causes it called spontaneous combustion.
3. Explosion :- when a crackers is ignited, a sudden reaction takes place with the evolution of heat, light and sound. The large amount of gas formed in the reaction is called explosion.
Flame :- an area of bright burning gas that comes from something that is on fire.
Structure of a flame :- the substance which vaporise during burning gives flame.
Ex. Kerosene oil and molten wax rise through the wick and are vaporized during burning and form flame.
* Charcoal does not vaporise and so does not produce a flame.
* There are 3 different zones of a flame dark zone, luminous zone and non luminous zone.
Fuel :- sources of heat energy for domestic and industrial purpose are mainly wood, charcoal, petrol, kerosene These substance are called fuels.
A good fuel is one which is readily available. It is cheap, it burns easily in air at a moderate rate, it does not leave behind any undesirable substances.
Ideal fuel :- an ideal fuel is cheap, readily available, readily combustible and easy to transport.
Fuel efficiency :- fuel efficiency is expressed in terms of its calorific value which is expressed in units of kilo joule per kg.
Calorific value :- the amount of heat energy produced on complete combustion of 1 kg of a fuel is called its calorific value.
The calorific value of a fuel is expressed in a unit called kilo joule per kg.
Types of fuel :-
1. Solid fuel
2. Liquid fuel
3. Gaseous fuel
1. Solid fuel :- combustible substance which are solid at room temperature called solid fuel.
Ex. Coal, coke, wood charcoal etc.
2. Liquid fuel :- volatile liquids which produce combustible e vapour called liquid fuel.
Ex. Petrol, kerosene, alcohol, diesel etc.
3. Gaseous fuel :- combustible gases or mixture of combustible gases.
Ex. Natural gas, LPG, CNG, biogas etc.
Burning of fuels leads to harmful product :- the increasing fuel consumption has harmful effect on the environment.
** carbon fuels like wood, coal, petroleum release unburn carbon particles. These fine particles are dangerous pollutants causing respiratory disease such as asthma.
**
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