CBSE Class 8 Science Combustion and Flame Chapter Notes

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The most important source of energy is fuels. Flame is a region where combustion (or burning) of gasesous substance takes place. Most of the combustible substances on burning produce a flame. A flame is obtained by burning of vapours of the substances. Whenever a substance is burnt it produces heat. The heat is produced due to a chemical reaction. Let us study the chemical process of burning and the type of flame produced during this process.
What is combustion?
A chemical process in which of a substance reacts with oxygen to give off heat is called combustion. Combustion is the exothermic process. The substance that undergoes combustion is said to be combustible it is also called a fuel.

For example : Charcoal burns in air to give carbon dioxide and heat.
C + O2 → CO2 + Heat
Methane burns in air forming carbon dioxide. Water and heat.
CH4 + 2O2 → CO2 + 2H2O + Heat


Fix a lighted candle on a table. Put a glass chimeny over the candle and rest is on a few wooden blocks in such way that air can enter the chimney.


Observe what happens to the flame. Now remove the blocks and let the chimney rest on the table. Again observe the flame. Finally put a glass plate over the chimney. We find that for combustion air is necessary. The candle burns freely in case (a) when air can enter the chimney from below. In case (b) when air does not enter the chimney from below, the flame flickers and produces smoke, in case (c) the flame finally goes off because the air is not available. This activity shows that air is necessary for combustion.

Combustible and Non combustible substances :
Substances such as paper, kerosene, petrol, straw and wood etc, which burns easily are known as combustible substances. Substances that don't burn easily are called non combustible substances. For example iron, glass, diamond etc.

Ignition temperature or kindling temperature :
The lowest temperature at which a substance starts burning is known as its kindling temperature or ignition temperature For example, the ignition temperature of white phosphorous is 35°C. Hence it catches fire on slight heating. The ignition temperature of red phosphorus is 260°C. Therefore, white phosphorus is kept immersed in water. Whereas red phosphorus is stored in ordinary bottles.

Make two paper cups by folding of paper pour about 150 ml of water in one of the cups. Heat both the cups separately with a candle


Conclusion : The heat supplied to the paper cup is transferred to water by conduction. So in the presence of water, the ignition temperature of paper is not reached. Hence it does not burn.
The substances which have very low ignition temperature and can easily catch fire with a flame are called inflammable substances. For example petrol, alcohol, LPG etc.

Condition necessary for combustion :
For combustion, the following three conditions are essential
(1) Presence of a combustible substance.
(2) Presence of a supporter of combustion.
(3) Attainment of ignition or kindling temperature.
How do we control fire? Water cools the combustion material so that its temperature is brought below its ignition temperature. Fire produced by the burning of oil or petrol cannot be controlled by throwing water on it because water being heavier than oil, settles down below the oil and oil continues to burn. In the case of fires caused by burning liquid fuels such as kerosene oil can be controlled by throwing sand or soil over it. There are various types of fire extinguishers used for controlling a fire. The job of a fire extinguisher is to cut of the supply of air or bring down the temperature of the fuel. Soda-acid fire extinguisher is based on the principle of cooling the fire. Carbon dioxide liberated by the reaction of sulphuric acid with sodium bicarbonate solution comes out with a stream of liquid water at high pressure. Water puts off the fire by lowering the temperature of the combustible material below its ignition temperature and carbon dioxide cuts off the supply of air to the combustible substance.


Types of combustion :
Combustion is mainly of three types
(1) Rapid Combustion (2) Spontaneous combustion (3) Explosion

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