Class 8 Combustion and Flame Notes

Combustion and Flame Notes


A chemical process in which a substance reacts with oxygen to give off heat is called combustion.

  • Sometimes, light is also given off during combustion, either as a flame or as a glow.

Ignition Temperature:-

The lowest temperature at which a substance catches fire is called its ignition temperature.

Inflammable Substance:-

The substances which have very low ignition temperature and can easily catch fire with a flame are called inflammable substances.
Ex:- Petrol, alcohol, Liquified Petroleum Gas (LPG), etc.

Conditions required for combustion:-

  • Combustible substance (Fuel)
  • Ignition Temperature
  • Oxygen

Fire control:-

Fire can be controlled by removing any or all of the factors of combustion, i.e. fuel, oxygen (air) and ignition temperature (by lowering the temperature).

Fire extinguisher:- It helps to cut off the supply of oxygen or bring down the temperature of the fuel or both.

Types of combustion:-

Slow Combustion:-

When a combustible substance burns at a slow, or moderate rate, its combustion is called slow combustion.
Ex:- Burning of cowdung cakes, wool, etc.

Rapid Combustion:-

When a combustible substance burns at a fast rate, its combustion is called rapid combustion.
Ex:- Combustion of gaseous fuel

Spontaneous Combustion:-

When a combustible substance catches fire on its own, even at room temperature, its combustion is called spontaneous combustion.
Ex:- Burning of phosphorus in air at room temperature

Explosive Combustion:-

The burning of a combustible material in a very short span of time in a closed space with the evolution of heat, light and sound.
Ex:- Burning of crackers


The gaseous substanes or the substances which vapourise during burning, give flames on burning.
For example,

  • Kerosene oil and molten wax are vapourised during burning and form flames.
  • Charcoal does not vapourise and so does not produce a flame.
fig:- Different Zones of Candle Flame

Combustible Substance/Fuel:-

The substance that undergoes combustion is said to be combustible substance or fuel.
Ex:- Wood, Kerosene, LPG (Liquified Petroleum Gas) etc.

  • The fuel may be solid, liquid or gas.

Characteristics of good fuel:-

  • It should be readily available.
  • It should be cheap.
  • It should burn easily in air at a moderate rate.
  • It should produce a large amount of heat.
  • It should leave no residue.

Calorific value:-

The amount of heat energy produced on complete combustion of 1 kg of a fuel is called its calorific value.

  • S.I. unit of calorific value:- Joule/Kg (J/kg)

Harmful effects of burning of fuels:-

  • The unburnt carbon particles produced by burning of carbon fuels like wood, coal, petroleum cause respiratory diseases, such as asthma.
  • Incomplete combustion of carbon fuels gives carbon monoxide gas which is a very poisonous gas.
  • Combustion of most fuels releases carbon dioxide that causes global warming.
  • Burning of coal and diesel releases sulphur dioxide gas. It is an extremely suffocating and corrosive gas.
  • Acid rain:- Oxides of sulphur produced by burning of coal and diesel and oxides of nitrogen produced by burning of petrol dissolve in rain water and give acid rain.
    Acid rain is very harmful for crops, buildings, soil and aquatic life.

Cleaner Fuel:-

The use of diesel and petrol as fuels in automobiles is being replaced by CNG (Compressed Natural Gas) because CNG produces harmful products and pollution in very less amounts. So, CNG is known as a cleaner fuel.


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