(a) Take some water in a paper cup and heat it. It is observe that it is possible to boil water without burning the paper cup. This is due to the reason that he heat given by the flame is quickly transferred from the paper cup to the water. As a result the temperature if the paper does not reach the ignition temperature and hence is not burned. The heat gained by water raises its temperature and finally it starts boiling.
(b) Take a piece of cotton cloth and soak it in a mixture containing equal amounts of water and alcohol. Now light this cloth with a match sticks. It is observed that the cloth catches fire immediately. After sometime, the fire extinguishes without burning the cloth.
The above observation can be easily explained as follows:
When the cloth soaked in alcohol and water is lighted. The alcohol catches fire immediately and starts burning. The heat produce due to combustion of alcohol is absorbed by water which gets converted into steam. The presence of water in the cloth does not allow the temperature of the cloth to rise above the boiling point of water. Since the ignition temperature of cloth is much higher than this temperature, therefore, it does not catch fire. When alcohol is completely consumed the wet cloth is left behind unburnt. Thus, the wet cloth does not burn because the presence of water keeps the temperature of the cloth ignition temperature.
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The substances which have low ignition temperature catch fire easily whereas the substances having high ignition temperature have to be heated to high temperature for starting the combustion and hence do not burn easily. Alcohol, petrol, kerosene, LPG, cotton, etc., has low ignition temperature and hence catches fire very easily. Coke and wood have relatively higher ignition temperatures. Out of petrol and kerosene, petrol has lower ignition temperature. Now you should be able to explain why adulterated petrol containi9ng kerosene as impurity, when used in vehicles, gives starting problem.
Now let us try to understand how a match stick is lighted. A match stick is lighted by rubbing it on the co0arse surface provided on the side of the match-box. The work done during rubbing generates heat due to friction. This heat raises the temperature of the chemical on the match stick head to its ignition temperature. The chemical, therefore, catches fire and the match stick starts burning.
A burning match stick can be used to light a candle. When a burning stick is brought near the weak of a candle, the temperature of the wax on the wick rises to ignition temperature it starts burning.
Combustion of wood which has relatively high ignition temperature, is generally started by pouring a little of kerosene over it and the lighting with a match stick. The burning match stick raises the temperature of kerosene to ignition temperature which, therefore, catches fire. This further heat up the wood to its ignition temperature and its combustion.