2. What is combustion ?
• A chemical process in which a substance
reacts with oxygen to give off heat is called
combustion. The substance that undergoes
is said to be combustible. It is also called a
fuel.
3. Try this activity
• Fix a lighted candle on a table. Put a glass chimney
over the candle and rest it on a few wooden blocks
in such a way that air can enter the chimney.
Observe what happens to the flame. finally, put a
glass plate over the chimney. Watch the flame
again. What happens in the three cases? Does the
flame flicker off? Does it flicker and give smoke?
Does it burn unaffected? Can you infer anything
at all about
4. Ignition temperature• You might have heard that when the clothes of a
person catch fire, the person is covered with a
blanket to extinguish fire. You must have had an
experience of burning a piece of paper. Does it
burn when a burning matchstick is brought near it?
Can you burn a piece of wood by bringing a lighted
matchstick near it? Why do you have to use paper
or kerosene oil to start fire in wood or coal? Do
these experiences tell you that different
substances catch fire at different temperatures?
The lowest temperature at which a substance
catches fire is called its ignition temperature.
5. Forest fire
• During extreme heat of summer, at some
places dry grasses catch fire. From grasses,
it spreads to trees, and very soon the whole
forest is on fire . It is very difficult to control
such fires.
6. History of matchstick
• The history of the matchstick is very old.
More than five thousand years ago small
pieces of pinewood dipped in sulphur were
used as matches in ancient Egypt. The
modern safety match was developed only
about two hundred years ago. 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.
7. Inflammable substances
• The substances which have very low ignition
temperature and can easily catch fire with a
flame are called inflammable substances.
8. How to control fire
• When a fire brigade arrives, what does it do?
It pours water on the fire .Water cools the
combustible material so that its temperature
is brought below its ignition temperature.
This prevents the fire from spreading. Water
vapors also surround the combustible
material, helping in cutting off the supply of
air. So, the fire is extinguished.
9. Fire extinguisher
• The most common fire extinguisher is water.
But water works only when things like wood
and paper are on fire. If electrical equipment
is on fire, water may conduct electricity and
harm those trying to douse the fire. Water is
also not suitable for fires involving oil and
petrol. Do you recall that water is heavier
than oil? So, it sinks below the oil, and oil
keeps burning on top
10. How do we get the supply of carbon dioxide? It can
be stored at high pressure as a liquid in cylinders. In
what form is the LPG stored in cylinders? When
released from the cylinder, CO2 expands
enormously in volume and cools down. So, it not
only forms a blanket around the fire, it also brings
down the temperature of the fuel. That is why it is
an excellent fire extinguisher. Another way to get
CO2 is to release a lot of dry powder of chemicals
like sodium bicarbonate (baking soda) or potassium
bicarbonate. Near the fire, these chemicals give off
CO2.
11. Types of combustion
• We find that the gas burns rapidly and produces
heat and light. Such combustion is known as rapid
combustion.
• The type of combustion in which a material
suddenly bursts into flames, without the application
of any apparent cause is called spontaneous
combustion.
12. Explosion
• A large amount of gas formed in the reaction
is liberated. Such a reaction is called
explosion. Explosion can also take place if
pressure is applied on the cracker.
13. Flame
• Observe an LPG flame. Can you tell the
colour of the flame. What is the colour of a
candle flame?
15. goldsmith
• Notice that the portion of the copper wire
just outside the flame gets red hot. Does it
indicate that the non-luminous zone of the
flame has a high temperature? In fact, this
part of the flame is the hottest part
Goldsmiths blow the outermost zone of a
flame with a metallic blow-pipe for melting
gold and silver
16. What is fuel?
• The sources of heat energy for domestic
and industrial purposes are mainly wood,
charcoal, petrol, kerosene, etc. These
substances are called fuels
17. Fuel efficiency
• Suppose you were asked to boil a given
quantity of water using cow dung, coal and
LPG as fuel. Which fuel would you prefer?
The amount of heat energy produced on
complete combustion of 1 kg of a fuel is
called its calorific value.
18. Burning of fuels leads to harming
products• Moreover cutting of trees leads to deforestation which is
quite harmful to the environment.
• Combustion of most fuels releases carbon dioxide in the
environment. Increased concentration of carbon dioxide in
the air is believed to cause global warming.
• Burning of coal and diesel releases sulphur dioxide gas. It is
an extremely suffocating and corrosive gas. Moreover,
petrol engines give off gaseous oxides of nitrogen. Oxides
of sulphur and nitrogen dissolve in rain water and form acids.
Such rain is called acid rain .
