Class 8 Chapter 6-Combustion and Flame Notes - Hakim.docx

Class 8 Chapter 6-Combustion and Flame Notes - Hakim.docx

1. Chapter 6- Combustion and Flame

We use different kinds of fuel for various purposes at home, in industry and for running
automobiles. These fuels are cow dung, wood, coal, charcoal, petrol, diesel, compressed
natural gas (CNG), etc.

Combustion: It is a chemical reaction in which heat is released by a material when it reacts
with oxygen.

Fuel or Combustible Substance: Any material that undergoes combustion is called a
combustible substance. It is also called as fuel. Some examples of fuels are petrol, diesel,
etc. The fuel may be in solid, liquid or gas state. Sometimes, light is also given off during
combustion, either as a flame or as a glow.
Ignition Temperature:
(i) It is the minimum temperature at which any material catches fire.
(ii) If the temperature of combustible substance is lower than the ignition temperature
then the substance will not burn.
Example: (i) Cooking oil catching fire when a frying pan is kept for long on a burning stove.
(ii) Kerosene oil and wood do not catch fire on their own at room temperature. But, if
kerosene oil is heated a little, it will catch fire. But if wood is heated a little, it would still
not catch fire.
Inflammable Substances:
Those materials which have low ignition temperature and catch fire easily are termed as
inflammable substances. Example includes petrol, LPG, etc.
Matchsticks:
Since ages, matchsticks are in use. Long ago, Egyptians used small pieces of pinewood
dipped in sulphur as matches. These days matchsticks are lot safer.

2. Modern matchsticks are made up with mixture of antimony trisulphide and potassium
chlorate with some glue and starch applied on the head of the match. The rubbing surface
has powdered glass and some red phosphorous. On striking match against rough surface,
red phosphorous gets converted into white phosphorous and it reacts with potassium
chlorate to ignite antimony trisulphate and so the combustion takes place.

Matchstick
Things necessary for combustion to take place:
(i) Fuel or Combustible substance.
(ii) Air (With presence of Oxygen in it).
(iii) Temperature above the Ignition temperature.

Measures to control fire:
1. Fire Brigade Stations:
In case of fire, fire brigades will extinguish the fire by sprinkling the water on the affected
areas. The water will bring down the temperature below its ignition temperature. As a
result, fire will stop spreading. Water vapour also surround the combustible material,
helping in cutting off the supply of air. So, the fire is extinguished.

3. 2. Fire Extinguisher:
Water is the most common fire extinguisher. But, it works only on things like wood, paper,
etc. However, in case fire is caught on electrical things then, water being good conductor
of electricity will destroy that equipment. Even water is not good in case of fires due to oil,
petrol, etc.
For such cases, Carbon dioxide (CO2) is best extinguisher. This extinguisher cut off the air
supply and thus brings down the temperature below the ignition temperature as a result
fire gets extinguished. Moreover, it usually does not damage electrical equipment.

3. Use of Blankets:
If a person catches the fire, then blankets can be used to extinguish the fire.

4. Forest Fires:
In summer season, when temperature rises too high then the regions having dry grasses

4. will catch the fire. This fire spreads rapidly from grasses to trees and eventually entire
forest is on fire. And it is difficult to manage such fires.

Different Types of Combustion:
1. Rapid Combustion
In this type of combustion, the substances burns rapidly and yield light and heat.

Example:Bring a burning matchstick or a gas lighter near a gas stove in the kitchen. Turn on
the knob of the gas stove. We find that the gas burns rapidly and produces heat and light.
2. Spontaneous Combustion:
In this type of combustion, substances burst out into flames suddenly without any known
reason.

5. Examples: Many disastrous fires in coal mines result due to this kind of combustion. The
heat rays coming from the sun or a lightning strike might be responsible for this kind of
combustion.
3. Explosion:
In this type of combustion, all of a sudden reaction results into heat, light and sound.
Moreover, large quantity of gas also gets released.

Example: When a fire cracker is ignited, a sudden reaction takes place with the evolution
of heat, light and sound with the large amount of gas.

6.4 Flame

 When something is burnt, a hot luminous gas emerges out of the substance. This gas
is called as flame.

 The substances which vapourise during burning, give flames. For example, kerosene
oil and molten wax rise through the wick and are vapourised during burning and
form flames. Charcoal, on the other hand, does not vapourise and so does not
produce a flame.

6. Flame structure:
When flames are observed carefully, one can notice different layers of flame as shown in
figure below:
 Outermost zone: It is blue in color and is hottest amongst all the zones. In this
portion, complete combustion takes place.
 Middle zone: It is yellow in color and is somewhat hot. In this portion, partial
combustion takes place.
 Innermost zone: It is black in color and is coolest amongst all the zones.

Table 6.2 Materials forming Flame on Burning

S.N
o.
Material Forms flame Does not form
flame
1. Candle
Yes
2. Magnesium
Yes
3. Camphor
Yes
4. Kerosene Stove
Yes
5. Charcoal
Yes

7.  In Activity 6.6, could the vapours of wax coming out of the glass tube be the cause of
the flame produced?

Activity 6.6

Light a candle (Caution: Be careful).
Hold a glass tube with a pair of
tongs and introduce its one end in
the dark zone of a non-flickering candle
flame [Fig. 6.10 (a)]. Bring a
lighted matchstick near the other
end of the glass tube. Do you see a
flame? If so, what is it that produces
a flame? Notice that the wax near
the heated wick melts quickly.







Fig. 6.10(b)

When the candle flame is steady,
introduce a clean glass plate/slide into
the luminous zone of the flame [Fig.
6.10 (b)]. Hold it there with a pair of
tongs for about 10 seconds. Then
remove it. What do you observe?

A circular blackish ring is formed on
the glass plate/slide. It indicates the
deposition of unburnt carbon particles
present in the luminous zone of the
flame.

8. Fig. 6.10(c)

Hold a thin long copper wire just
inside the flame for about 30 seconds
[Fig. 6.10 (c)].
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 [Fig. 6.10(d)].




Fig. 6.11

Goldsmiths blow the outermost zone
of a flame with a metallic blow-pipe formelting
gold and silver (Fig. 6.11). Why
do they use the outermost zone of the
flame?











Fuel:
The substance that undergoes combustion is called as fuel. Examples of fuels are wood,
charcoal, petrol, kerosene, etc.
Characteristics of good fuel:
(i) It should easily be available.
(ii) It should be cheap.
(iii) It should generate large amount of heat.
(iv) It should not leave any unwanted matter after combustion.

9. Ideal Fuel
(i) The fuel which satisfies all the characteristics of good fuel is termed as an ideal fuel.
(ii) Probably, there is as such no ideal fuel present.
Make a list of fuels familiar to you.
Group them as solid, liquid, and gaseous fuels as in Table 6.3.

Table 6.3 Types of Fuels

S. No. Solid Fuels Liquid Fuels Gaseous fuels
1. Coal Kerosene oil Natural gas
2.
Wood Petrol LPG
3.
Coke Diesel Biogas
Fuel Efficiency
 The amount of heat energy produced on complete combustion of 1 kg of a fuel is
called its calorific value. The more is the calorific value of a fuel, more is the
efficiency of the fuel.
 The calorific value of a fuel is expressed in a unit called kilojoule per kg (kJ/kg).

Calorific values of some fuels are given in Table 6.4.

10. Burning of Fuels Leads to Harmful Products

The increasing fuel consumption has harmful effects on the environment.

1. Carbon fuels like wood, coal, petroleum release unburnt carbon particles. These fine
particles are dangerous pollutants causing respiratory diseases, such as asthma.

2. Incomplete combustion of these fuels gives carbon monoxide gas. It is a very poisonous
gas. It is dangerous to burn coal in a closed room. The carbon monoxide gas produced can
kill persons sleeping in that room.

3. Combustion of most fuels releases carbon dioxide in the environment. Increased
concentration of carbon dioxide in the air is believed to cause global warming.

4. 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. It is
very harmful for crops, buildings and soil.










Global Warming Acids Rain

The use of diesel and petrol as fuels in automobiles is being replaced by CNG
(Compressed Natural Gas), because CNG produces the harmful products in very small
amounts. CNG is a cleaner fuel.

11. CNG Auto Mobiles

12. NCERT Solution for Combustion and Flame
QUESTIONS FROM TEXTBOOK
Q.1. List conditions under which combustion can take place. Ans. Conditions necessary for
combustion are:
(i) Presence of a combustible substance.
(ii) Attainment of ignition temperature.
(iii) Proper supply of air to provide oxygen.
Q.2. Fill in the blanks.
(a) Burning of wood and coal causes _________ of air.
(b) A liquid fuel used in homes is _________.
(c) Fuel must be heated to its_________before it starts burning.
(d) Fire produced by oil cannot be controlled by_________.
Ans. (a) pollution (b) kerosene.
(c) ignition temperature (d) water.
Q.3. Explain how the use of CNG in automobiles has reduced pollution in our cities. The
use of CNG in place of petrol and diesel reduce pollutions in following ways:
(i) It produces less carbon monoxide gas.
(ii) It produces less carbon dioxide gas.
(iii) It produces less amount of sulphur dioxide and nitrogen dioxide which cause acid rain.
(iv) No residue remains after combustion.
Q.4. Compare LPG and wood as fuels.
Ans. Differences:

13. Q.5. Give reasons:
(a) Water is not used to control fires involving electrical equipment.
(b) LPG is a better domestic fuel than wood.
(c) Paper by itself catches fire easily whereas a piece of paper wrapped around an
aluminium pipe does not.
Ans. (a) Water is a good conductor of electricity. It conducts electricity and may result
electric shock.
(b) LPG has more calorific value and produces no pollution. So it is better domestic
fuel than wood.
(c) The ignition temperature of paper is less, so it catches fire easily. It does not
catch fire when wrapped around aluminium pipe because aluminium absorbs the heat, so
paper does not attain its ignition temperature.
Q.6. Make a labelled diagram of candle flame.
Ans.

Fig. 6.2 Different zones of Candle flame
Q.7. Name the unit in which the calorific value of a fuel is expressed.
Ans. Kilojoules per kg (kJ/kg)
Q.8. Explain how CO2 is able to control fires.
Ans. (i) CO2 forms a blanket around fire due to which supply of air is stopped.
(ii) CO2 also brings down the temperature of the fuel.
Q.9. It is difficult to burn a heap of green leaves but dry leaves catch fire easily. Explain.
Ans. The green leaves contain some water due to which the ignition temperature of leaves
increases and they do not catch fire easily while dry leaves have no water, so they catch
fire easily.

14. Q.10. Which zone of a flame does a goldsmith use for melting gold and silver and why?
Ans. A goldsmith uses the outer zone (non-luminous zone) of a candle flame to melt gold
and silver because it is the hottest zone and has more temperature.
Q.11. In an experiment 4.5 kg of a fuel was completely burnt. The heat produced was
measured to be 180,000 kJ. Calculate the calorific value of the fuel.
Ans. Total mass of fuel = 4.5 kg
Total heat produced = 180,000 kJ
Heat produced by burning 1 kg of fuel = 180,000 kJ/4.5 kg = 40,000 kJ/kg.
So, calorific value of fuel = 40,000 kJ/kg.
Q.12. Can the process of rusting be called combustion? Discuss.
Ans. The process of rusting cannot be called combustion because in this process no
heat and light is produced. Due to this reason iron is not considered as combustible
substance.
Q.13. Abida and Ramesh were doing an experiment in which water was to be heated in a
beaker. Abida kept the beaker near the wick in the yellow part of the candle flame.
Ramesh kept the beaker in the outermost part of the flame. Whose water will get heated
in a shorter time?
Ans. The water heated by Ramesh will get heated in a shorter time because he kept
his beaker near the hottest zone of the flame.

15. Important Questions for Class 8 Science Chapter 6 – Combustion and Flame PDF

17. CBSE Class 8 Science Chapter 6 MCQ Type Questions
1. Which among the following is a fuel?
(a) Solid, Liquid or gas
(b) Gas only
(c) Liquid only
(d) Solid only
2. What type of reaction is a combustion?
(a) exothermic
(b) endothermic
(c) volatile
(d) none of these
3. Large scale cutting down of trees could result in ___
(a) afforestation
(b) deforestation
(c) both afforestation and deforestation
(d) reforestation

18. 4. What is burning of LPG an example of?
(a) spontaneous combustion
(b) slow combustion
(c) rapid combustion
(d) both slow and spontaneous combustion

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