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ii. Some substances have a high ignition temperature. They can catch fire only on
strong heating. For example Coal, Charcoal, Log of wood, etc.

*A paper cup containing water if shown over fire does not burn, whereas an empty
paper cup burns. This can be explained as, when we heat the empty paper cup, the
ignition temperature of the paper cup is reached very quickly and it starts burning.

But when we heat the paper containing water, then the heat supplied to the paper is
transferred to the water inside it by conduction. Due to the continuous transfer of heat,
the paper cup does not get heated too much to reach its ignition temperature.

The History of Matchstick
More than a thousand years ago small pieces of pinewood dipped in sulphur were used as
matches in ancient Egypt. The modern safety match was developed 200 years ago.
A mixture of antimony trisulphide, potassium chlorate, and white phosphorus with some
glue and starch was applied on the head of a match made up of suitable wood. When
struck against a rough surface, white phosphorus got ignited due to the heat of friction.
This started the combustion of the match. However, white phosphorus proved to be
dangerous both for the workers involved in the manufacturing of the matches and for the
users.
These days the head of the safety match contains only antimony trisulphide and potassium
chlorate. The rubbing surface has powdered glass and a little red phosphorus. When the
match is struck against the rubbing surface, some red phosphorus gets converted into
white phosphorus. This immediately reacts with potassium chlorate in the matchstick head
to produce enough heat to ignite antimony trisulphide and start the combustion.
How can we control fire?
To control fire, any of the three conditions that are necessary for burning can be removed.
Thus, a fire can be extinguished in three ways.
1. By removing the fuel (combustible substance).
Suppose fire starts in a room, all the combustible substances like furniture clothes, etc
should be removed so that the fire may not spread and the fire can be controlled.
2. By removing the heat (or by lowering the temperature)
Water is used to remove the heat or to lower the temperature of a burning substance
and to make it too cool, to burn further. It is the most common fire extinguisher. Water
cools the combustible substance below its ignition temperature and it stops burning.
The fire produced by burning oil and petrol cannot be extinguished by using water as
water is heavier than oil and petrol. So, when water is thrown over oil and petrol it settles
down. The oil floats on water and continues to burn.
The fire caused by an electrical short circuit cannot be extinguished by using water as
ordinary water conducts electricity to some extent and it can give an electric shock to the
person involved in fire-fighting
3. By cutting off the air supply.
Many fires can be extinguished by cutting off the air supply to the burning substances.
The air supply can be cut off by covering the burning substance with carbon dioxide,
sand, a blanket or a damp cloth etc.
Carbon dioxide is heavier than oxygen. It can be stored as a liquid at high pressure.
When released it spreads over the fire and extinguishes it. Another way to obtain carbon
dioxide is to release a lot of dry powder of chemicals like sodium bicarbonate or

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