This is a ppt on class 8 chapter 6 Combustion and Flame

1. COMBUSTION AND
FLAME

2. COMBUSTION
 What is Combustion?
A chemical process in which a substance
reacts with oxygen to give of heat and light is
called combustion.
 Examples:
Combustion of Magnesium
Mg + O2  MgO + Heat + Light

3. COMBUSTIBLE AND NON-COMBUSTIBLE
 Those substance which can burn are
called combustible. Eg. Paper, Cloth,
Straw, Petrol, etc.
 Those substance which cannot burn are
called non-combustible. Eg. Stone, glass,
cement, bricks, soil, etc.

4. CONDITIONS NECESSARY FOR COMBUSTION
 Presence of a combustible substance.
 Presence of a supporter of combustion.
 Heating the combustible substance to its
ignition temperature.
Let us read them one by one.

5. COMBUSTIBLE SUBSTANCE
 The presence of a combustible substance is
necessary for combustion to take place.
 A combustible substance is actually the “food
of fire”

6. SUPPORTER OF COMBUSTION
 The most common supporter is something
which is all around us “AIR”.
 So, we can say that air is necessary for
combustion.
 E.g. When the clothes of a person catches
fire then the person is covered with a
blanket. The blanket stops the supply of AIR
to the clothes which had caught fire.

7. IGNITION TEMPERATURE
 Before a combustible substance can catch
fire and burn, it must be heated in its
minimum temperature so that the
substance can catch fire.
 This MINIMUM TEMPERATURE is the
ignition temperature.
 Hence, The lowest temperature at which a
substance catches fire and starts burning
is called its IGNITION TEMPERATURE.

8. IGNITION TEMPERATURE
 Some substances have low IGNITION
TEMPERATURE: Paper, Splinter of wood,
Dry Grass etc.
 The substances which have very low
ignition temperatures and can easily
catch fire with a flame are called
inflammable substances.

9. IGNITION TEMPERATURE
 How a matchstick is lighted?
 A matchstick starts burning on rubbing it on
the side of the matchbox because the heat
produced by friction heats the chemicals at
the head of the matchstick to their ignition
temperature and make it catch fire.

10. HOW DO WE CONTROL FIRE
 We know that Fire is a good servant but a
bad master.
 Any fire needs 3 things to be present: Fuel,
Heat, Air(Oxygen); If any one of these 3
things is removed, then the burning will stop
and fire will be extinguished.
 Thus a fire can be stopped by 3 ways:
1. By removing the Fuel.
2. By removing the Heat.
3. By cutting of the air supply.

11. REMOVE THE FUEL
 A fuel is a food for fire. So, when fire start in
a room, all the combustible substances like
furniture, clothes, and books, etc., should be
removed at once so that fire may not spread.
 It is, however, not possible remove all the
combustible materials from the place of fire

12. REMOVE THE HEAT
 Water is used to remove heat from a burning
substance and to make it too cool to burn further.
Water is a very common fire extinguisher that can
extinguish common fire only.
 When water is thrown on a burning substance, it gets
cooled below its ignition temperature and stops
burning.
 But still fire produced by oil, petrol or any electrical
appliance cannot be extinguished with water. Oil and
petrol floats on water and hence the fire would spread
more and more. The person trying to extinguish
fire(due to electrical appliance) might get an electric
shock.

13. CUT OFF THE AIR SUPPLY
 AIR[Oxygen] is a supporter of fire and hence
if the air supply is cut-off then the fire would
stop.
 The best way to extinguish fire is to
extinguish fire with a FIRE EXTINGUISHER.
 It produces carbon dioxide which stops fire.

14. TYPES OF COMBUSTION
 There are three types of COMBUSTION
 RAPID Combustion
 SPONTANEOUS Combustion
 EXPLOSION Combustion

15. RAPID COMBUSTION
 The combustion reaction in which a large
amount of heat and light are produced in a
short time is called RAPID Combustion.
 The immediate burning of cooking gas in a
gas stove to give off heat and light, is an
example of RAPID Combustion.

16. SPONTANEOUS COMBUSTION
 The combustion reaction which occurs on
its own (without any help of external heat)
is called SPONTANEOUS Combustion.
 The burning of white phosphorus at its own
room temperature is an example of
SPONTANEOUS Combustion

17. EXPLOSIVE COMBUSTION
 A very fast combustion reaction in which a
large amount of heat, light and sound are
produced, is called EXPLOSIVE
Combustion.
 The fireworks which we explode during
festivals are based on the EXPLOSIVE
Combustion

18. FLAME
 A FLAME is a region where combustion of
gaseous substances takes place.
 LPG, biogas, wax, camphor, magnesium,
kerosene oil and mustard oil produce a flame
when burnt. Coal, Charcoal etc. does not
produce any flame when burnt.

19. STRUCTURE OF A FLAME
Let us understand these one by one

20. THE INNERMOST REGION
 The innermost region is black. This zone
consists of hot unburnt vapour of the
combustible material. The innermost zone is
the least hot part of the flame.

21. THE MIDDLE ZONE
 The middle zone of the flame is yellow. It is
bright and luminous. The fuel vapors burns
partially in the middle zone because there is
not enough air for burning this zone.

22. THE OUTERMOST ZONE
 The outermost zone of a flame is blue. It is
non-luminous zone. In the outer zone
complete combustion of the fuel takes place
because there is plenty of air in the
surrounding.

23. FUEL EFFICIENCY
 Calorific Value:
Different fuels produce different amount of
heat on burning. The amount of heat
produced by the complete burning of 1
kilogram of a fuel is called CALORIFIC
VALUE.
 The unit used for this is known as Kilojoules
per kilogram(kj/kg).

24. CALORIFIC VALUE OF FUELS
FUEL CALORIFIC VALUE
1. Cow-dung cakes 6000 to 8000 kJ/kg
2. Wood 17000 to 22000 kJ/kg
3. Coal 25000 to 33000 kJ/kg
4. Biogas 35000 to 40000 kJ/kg
5. Petrol 45000 kJ/kg
6. Kerosene 45000 kJ/kg
7. Diesel 45000 kJ/kg
8. Methane 50000 kJ/kg
9. CNG 50000 kJ/kg
10. LPG 55000 kJ/kg
11. Hydrogen Gas 150000 kJ/kg

25. IDEAL FUEL
 An ideal fuel has the following characteristics
I. It has a high calorific value.
II. It burns easily in air at a moderate rate.
III. It has a proper ignition temperature.
IV. It does not produce any harmful gases
or leaves any residue after burning.
 Please note that there is perhaps no fuel
that can be considered as an ideal fuel

26. FLAME
 Goldsmiths blow air with a blow pipe to
intensify a kerosene lamp flame for melting
and moulding the pieces of gold and silver
into desired pieces to make jewellery. When
air is blown through blow-pipe into the flame,
it helps in the combustion of unburnt fuel and
hence makes the flame hotter.

27. BURNING OF FUELS PRODUCES HARMFUL PRODUCTS
1. The burning of fuels like wood, coal and
petroleum products releases unburnt carbon
particles in the air.
2. Incomplete combustion of fuels(due to
insufficient air) produces a very poisonous gas
carbon monoxide.
3. Burning of fuels releases carbon dioxide into
air in the environment.
4. Burning of coal, petrol and diesel produces
sulphur dioxide gas which goes into the air.

28. THE CASE OF CNG
 The use of petrol and diesel as fuels in
automobiles is being replaced by
CNG(Compressed natural gas). This is
because when CNG burns, it produces very
small amount of harmful gases. CNG is clean
fuel because it burns without producing
smoke. Since the burning of CNG much less
harmful products and smoke, the use of CNG
as fuel in automobiles has reduced air
pollution in our cities.

29. THE CASE OF WOOD
 Wood has been used as a domestic and
industrial for centuries. People still use wood
sometimes. Still there are some
disadvantages of wood as fuel:
 The burning of wood causes smoke.
 The cutting down of wood from trees causes
deforestation.

30. THE CASE OF WOOD
 Wood is being replaced with LPG.
 LPG is a better domestic fuel than wood as
fuel due to following reasons:
 LPG has a much higher calorific value than
wood.
 LPG burns without producing any smoke.
 LPG burns completely without leaving behind
any solid residue.
 LPG is easy to transport.

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