Question

with the help of an experiment explain that air is necessary for a substance to undergo combustion.

Answer 1

candle burning experiment

Answer 2

There are three conditions which are necessary for combustion to take place:

 1)Presence of combustible substance
2)Presence of supporter of combustion
3)Heating the combustible substance to its ignition temperature
Presence of combustible substance
The presence of combustible substance is necessary for combustion to take place.It is actually the food for fire.
Presence of supporter of combustion
The most common supporter of combustion which we have around us is air.
For Ex:
1)If burning charcoal is covered with a vessel,it stops burning after some time,ie,the charcoal fire gets extinguished after some time.Actually when we cover the burning charcoal with a vessel,the supply of supporter of combustion to the burning charcoal is cut off and hence the charcoal fire stops.
2)When the clothes of person catch fire,the person is covered with a blanket to extinguish the fire.This is because when the burning clothes of a person are covered with a blanket,the supply of air to the burning clothes is cut off due to which the clothes stop burning-the fire gets extinguishes.
We find that when the supply of supporter of combustion to a burning substance is cut off,then the process of burning also stops.

 3)Ignition Temperature
The lowest temperature at which a substance catches fires and starts burning is called ignition temperature.
It is necessary to heat a combustible substance to its ignition temperature so that it may undergo combustion
For Ex:The ignition temperature of paper is 233 °C.This means that a paper has to be heated at least to a temperature of 233 °C so that it may catch fire and start burning.
A combustible substance cannot catch fire as long as its temperature is lower than its ignition temperature.
A piece of paper does not catch fire at the room temperature because the ignition temperature of paper is much higher than the room temperature.
When we apply a burning matchstick to the piece of paper,it starts burning because the heat produced by burning matchstick heats the piece of paper to its ignition temperature and makes it burn.
The ignition temperature of different substances are different .
1)Some of the substances have low ignition temperature.
For Ex:Paper,splinter of wood, straw,cloth,alcohol,kerosene,biogas,LPG,CNG etc.
The substances which have very low ignition temperature and can easily catch fire with a flame are called inflammable substance.These substances are very dangerous to use.

 Petrol has a much lower ignition temperature than that of kerosene.Due to its very low ignition temperature,petrol catches fire easily on being lightened with a matchstick and burns explosively.That is why it is not used in stoves.
Kerosene has a higher ignition temperature due to which it burns smoothly in kerosene stove.
2)Some of the substances have high ignition temperature.
For Ex:Coal,charcoal,log of wood, cow-dung.
The substances which have very high ignition temperature and cannot easily catch fire with a flame are called flammable substance.