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Combustion and Flame - Class 8 : Notes


We use different kinds of fuel for various purposes at home, in industry and for running automobiles.These fuels are cowdung, wood, coal, charcoal, petrol, diesel, compressed natural gas (CNG), etc.

Combustion: 
It is a chemical reaction in which heat is released by a material when it reacts with oxygen.

Fuel or Combustible Substance:Any material that undergoes combustion is called a combustible substance. It is also called as fuel. Some examples of fuels are petrol, diesel, etc. The fuel may be in solid, liquid or gas state. Sometimes, light is also given off during combustion, either as a flame or as a glow.

Ignition Temperature:
(i) It is the minimum temperature at which any material catches fire.
(ii) If the temperature of combustible substance is lower than the ignition temperature then the substance will not burn.
Example: (i) Cooking oil catching fire when a frying pan is kept for long on a burning stove.
(ii) Kerosene oil and wood do not catch fire on their own at room temperature. But, if kerosene oil is heated a little, it will catch fire. But if wood is heated a little, it would still not catch fire.

Inflammable Substances:
Those materials which have low ignition temperature and catch fire easily are termed as inflammable substances. Example includes petrol, LPG, etc.

Matchsticks:
Since ages, matchsticks are in use. Long ago, Egyptians used small pieces of pinewood dipped in sulphur as matches. These days matchsticks are lot safer.
Modern matchsticks are made up with mixture of antimony trisulphide and potassium chlorate with some glue and starch applied on the head of the match. The rubbing surface has powdered glass and some red phosphorous. On striking match against rough surface, red phosphorous gets converted into white phosphorous and it reacts with potassium chlorate to ignite antimony trisulphate and so the combustion takes place.

Matchstick

Things necessary for combustion to take place:
(i) Fuel or Combustible substance.
(ii) Air (With presence of Oxygen in it).
(iii) Temperature above the Ignition temperature.
Measures to control fire:
1. Fire Brigade Stations:
In case of fire, fire brigades will extinguish the fire by sprinkling the water on the affected areas. The water will bring down the temperature below its ignition temperature. As a result, fire will stop spreading. Water vapours also surround the combustible material, helping in cutting off the supply of air. So, the fire is extinguished.

2. Fire Extinguisher:
Water is the most common fire extinguisher. But, it works only on things like wood, paper, etc. However, in case fire is caught on electrical things then, water being good conductor of electricity will destroy those equipment. Even water is not good in case of fires due to oil, petrol, etc.
For such cases, Carbon dioxide (CO2) is best extinguisher. This extinguisher cut off the air supply and thus brings down the temperature below the ignition temperature as a result fire gets extinguished. Moreover, it usually does not damage electrical equipment.

3. Use of Blankets:
If a person catches the fire, then blankets can be used to extinguish the fire.

4. Forest Fires:
In summer season, when temperature rises too high then the regions having dry grasses will catch the fire. This fire spreads rapidly from grasses to trees and eventually entire forest is on fire. And it is difficult to manage such fires.


Different Types of Combustion:
1. Rapid Combustion
In this type of combustion, the substances burns rapidly and yield light and heat.

Example:Bring a burning matchstick or a gas lighter near a gas stove in the kitchen. Turn on the knob of the gas stove. We find that the gas burns rapidly and produces heat and light.

2. Spontaneous Combustion:
In this type of combustion, substances burst out into flames suddenly without any known reason.

Examples: Many disastrous fires in coal mines result due to this kind of combustion. The heat rays coming from the sun or a lightning strike might be responsible for this kind of combustion.

3. Explosion:
In this type of combustion, all of a sudden reaction results into heat, light and sound. Moreover, large quantity of gas also gets released.

Example: When a fire cracker is ignited, a sudden reaction takes place with the evolution of heat, light and sound with the large amount of gas.


Flame:

When something is burnt, a hot luminous gas emerges out of the substance. This gas is called as flame.Flames are result of the substances which vaporizes on burning. Example includes kerosene oil, wax, etc. which form flames on burning.

Flame structure:
When flames are observed carefully, one can notice different layers of flame as shown in figure below:

Outermost zone: It is blue in color and is hottest amongst all the zones. In this portion, complete combustion takes place.
Middle zone: It is yellow in color and is somewhat hot. In this portion, partial combustion takes place.
Innermost zone: It is black in color and is coolest amongst all the zones.

 

Fuel:
The substance that undergoes combustion is called as fuel. Examples of fuels are wood, charcoal, petrol, kerosene, etc.

Characteristics of good fuel:
(i) It should easily be available.

(ii) It should be cheap.
(iii) It should generate large amount of heat.
(iv) It should not leave any unwanted matter after combustion.

Ideal Fuel
(i) The fuel which satisfies all the characteristics of good fuel is termed as an ideal fuel.

(ii) Probably, there is as such no ideal fuel present.

Fuel Efficiency
(i) The quantity of heat generated on combustion of 1 kg of a fuel is called its calorific value.
(ii) Its unit is kilojoule per kg (kj/kg).

Calorific Values of Different Fuels:

Fuel

Calorific Value

(kJ/kg)

Cow dung cake 6000 - 8000
Wood 17000 - 22000
Coal 25000 - 33000
Petrol 45000
Kerosene 45000
Diesel 45000
Methane 50000
CNG 50000
LPG 55000
Biogas 35000 – 40000
Hydrogen 150000

Harmful Effects of Burning Fuels: The increasing fuel consumption has harmful effects on the environment.
1. Various air pollutants like unburnt carbon particles released due to partial burning of carbon fuels causes many respiratory diseases.
2. The partial burning of some fuels releases carbon monoxide, which is a poisonous gas. And this gas can kill a person if left in a room filled with this gas.
3. Global Warming: Combustion of most fuels the increase the amount of carbon dioxide in the atmosphere that has lead to increase in the average temperature on the earth.


4. Acid Rain: Due to burning of coal and diesel, Chemicals like sulphur dioxide  and nitrogen dioxide are released into the air. The pollutants reacts with the water vapour present in the air and forms sulphuric and nitric acid. When it rains, these acids are also present. Such kind of rain is called Acid Rain.  It is very harmful for crops, buildings and soil.

Acids Rain

Prevention from Acid rain: The use of diesel and petrol as fuels in automobiles is being replaced by CNG (Compressed Natural Gas), because CNG produces the harmful products in very small amounts. CNG is a cleaner fuel.

CNG Auto Mobiles


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