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Combustion and Flame - Class 8 : Notes




Meaning of Combustion

  • A chemical process in which a substance reacts with oxygen to give off heat is called combustion.
  • The substance that undergoes combustion is said to be combustible substances or fuel.
  • Fuel releases its energy through a chemical reaction. It is a type of oxidation reaction where the fuel reacts with oxygen.
  • The fuel may be solid,liquid or gas.
  • Some examples of combustion are given below:-
    • When we burn charcoal in air,it produces carbon dioxide,heat and light.
      burning charcoal in air,it produces carbon dioxide,heat and light
      burning charcoal in air,it produces carbon dioxide,heat and light
      Here since oxygen is necessary for combustion to take place it is acting as supporter of combustion.
    • Food is a source of energy for our bodies. During the process of respiration, the digested food (such as glucose) is broken down in the body cells by a reaction with oxygen, resulting in carbon dioxide, water, and heat energy. Our bodies make use of this heat energy. Thus, respiration is a slow combustion of food that occurs in the body to generate heat energy.
    • The process of rusting can be called combustion as rusting also takes place slowly in the air in presence of humidity in the atmosphere. It is also a slow combustion process where small amount of heat is liberated and there is no production of light.

Combustible Substances and fuel

  • A fuel is a substance that is burnt to provide heat .
  • The substances which burn easily and causes a fire are called combustible or inflammable substances.
  • Fuel come in different forms. Fuels are of different types such as
    1. wood
    2. coal
    3. charcoal
    4. petrol
    5. diesel
    6. Compressed natural gas(CNG) and
    7. liquified petroleum gas (LPG)
  • Petrol is a highly combustible substance and the most commonly used inflammable substance.

Classification of fuel

  • Fuel can be classified as natural (primary) fuels and derived (secondary) fuels.
  • If a fuel is present in it’s natural state ,it is called natural fuel.
  • If a fuel is processed to improve its quality,it is called derived fuel.

Primary fuels are classified into three types

  1. Solid(Wood)
  2. Liquid(Crude oil)
  3. Gas(Natural gas)

Secondary fuels or derived fuels are classified into three types as

  1. Solid(Coke)
  2. Liquid(Kerosene)
  3. Gas(Coal gas)

Non-Combustible substances

table showing examples combustible and non-combustible substances
table showing examples combustible and non-combustible substances

  • The substances which don’t burn are called non-combustible substances or non-inflammable substances. For example -Stone,Glass,Iron nails.

Uses of fuel

  • Different types of fuels are used for different purposes such as cooking,running automobiles and for manufacturing goods.
  • LPG is considered as a better domestic fuel than wood or coal.
  • LPG is a considered as a better domestic fuel because of the following reasons
    1. LPG has more calorific value than wood or coal.
    2. LPG produces no pollution. Coal is similar to wood and also produces smoke.

Fuel efficiency

  • The efficiency of various fuels can be compared by the amount of heat produced by them.
  • In other words, a fuel is rated by its calorific value.

Calorific Value

  • The amount of heat energy released by the complete combustion of 1kg of a fuel.
  • The calorific value is expressed in kilojoule/Kg.
  • A fuel with more calorific value releases more energy than the fuel with lower calorific value and vice-versa.

Calorific values of various fuels


Harmful effects of burning fuels

  • Burning of wood and coal releases unburnt carbon particles which cause asthma .
  • Burning of most fuels releases carbon dioxide in the environment .Increased concentration of carbon dioxide in the air cause global warming.
  • Burning of Petrol,Diesel produces sulphur dioxide and also vehicles produce nitrogen dioxide.These reach the atmosphere, mix with rain water in clouds and form acids.This rain is called acid rain which is harmful for crops,building and soil.
  • Incomplete combustion of fuel produces carbon monoxide which is a very poisonous gas which can even led to the death of a person.
  • Ideal fuel

    Ideal fuel is a fuel which on burning doesn’t emit harmful gases ,available at a cheaper price,and also has a high calorific value.For example- Natural gas, LPG,CNG.

    Characteristics of ideal fuel

    • It should be cheap.
    • It should be readily available.
    • It should burn easily at a moderate rate.
    • It should have a high calorific value.
    • It should be less polluting.
    • It should be easy to handle,store and transport.

    Types of Combustion

    Combustion can be classified into three types namely

    1. Rapid combustion- When a substance catches fire instantly and produces heat and light in a very short time.For example- Lighting up of LPG gas stove in the kitchen.
    2. Spontaneous Combustion-When a substance does not need external heat to start the Combustion,and it catches fire suddenly on it’s own.For example- Forest fires. one other such example is spontaneous burning of phosphorus at room temperature.
    3. Explosion- An explosion is a combustion in which a sudden reaction takes place along with the evolution of heat,light and sound.For example- explosion of cracker on applying heat and pressure.

    types of combustion concept map
    Types of combustion concept map

    List the conditions under which combustion can take place

    • Presence of combustible substance.
    • Presence of supporter of combustion.
    • Attainment of Ignition temperature.

    Question. What is the supporter of Combustion?
    Answer. Any substance or medium which supports Combustion is called supporter of Combustion.Example- Oxygen.

    Ignition Temperature

    The lowest temperature at which a substance catches fire is called its ignition temperature. The substances with low ignition temperature will catch fire easily whereas the substances with higher ignition temperature will take longer to burn.

    ignition temperatire of different materials

    How can we control fire?

    We can prevent a substance from catching fire by not allowing its temperature to reach the ignition temperature or by cutting off the air supply or removing the fuel. This can be done by making use of:-

    • Water: A water extinguisher can put out things like burning wood,paper or a ordinary fire but doesnot work well with electrical fire as water conducts electricity. Water cannot be used to put out fire caused by inflammable substances like petrol, oil as water float on water and continue to burn.
    • Fire Extinguisher: Made up of metal and are filled with highly compressed carbon dioxide.The function of a fire extinguisher is to cut off the supply of air.It is possible because carbon dioxide being heavier displaces oxygen and surrounds the burning fuel.
    • Use of blankets: Putting a blanket over a person whose clothes are on fire break the contact of air (oxygen).

    How can we control fire?

    Flame

    A flame is the region where combustion of fuel takes place.The flame is produced by the substances that vaporizes easily on burning. The coloured of the flame depends on the temperature of the flame and the nature of substance that is burning.
    For example

    1. Burning of a candle produces flame.
    2. On lightning the cooking gas stove a bluish flame can be seen.

    Study of a candle flame

    There are three zones present in a candle flame when candle flame is observed carefully .

    Candle Flame

    1. Dark innermost zone or zone of no combustion-This zone consists of unburnt wax vapors.It appears dark black and is the least hot region.
    2. Luminous middle zone or zone of incomplete combustion (Yellow flame formed)-In this zone,the wax vapours starts burning. The heat produced converts the wax particles into carbon particles and when these particles get extremely hot they emit yellow light.It is moderately hot.
    3. Non-Luminous outer zone or zone of complete combustion(Blue flame formed)-Due to sufficient supply of oxygen from surroundings ,there is a complete Combustion of wax vapours takes place and the flame appears blue .It is the hottest part.

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