List conditions under which combustion can take place.
There are certain
conditions required for combustion to take place. They are:
(i) Presence of a fuel
(ii) Air (or oxygen)
(iii) Ignition temperature (minimum
temperature at which a substance catches fire)
Fill in the blanks:
a) Burning of wood and coal causes
__pollution__ of air.
(b) A liquid fuel used in homes is
__liquefied petroleum gas (LPG) _.
(c) Fuel must be heated to its
__ignition temperature__ before it starts burning.
(d) Fire produced by oil cannot be
controlled by __water__.
Explain how the use of CNG in automobiles has reduced pollution in our cities.
ANSWER:-Combustion of fuels like petroleum
causes formation of un-burnt carbon particles along with carbon monoxide gas.
These harmful pollutants enter the air and cause respiratory diseases.
Compressed Natural Gas (CNG) produces
these harmful products in very less quantity. It is a comparatively cleaner
fuel. Therefore, the use of CNG has reduced pollution in our cities.
Compare LPG and wood as fuels.
LPG:-1. LPG is a costly fuel but readily
available, combustible and
easy to transport in cylinders and tanker.
2. LPG is more energy efficient
3. Its calorific value is 55000 kJ/ kg
4. It causes less air pollution and prevents
deforestation by
supplementing the fuel need in place of wood as fuel
5. It is an exhaustible natural
as fuel-:
1. Wood
is a cheap fuel, not readily available in

cities and not readily combustible.
2. Wood as Fuel in not much energy
3. Its calorific value is 17000-22000
kJ/ kg
4. It causes air pollution and deforestation.
5. It is not an exhaustible natural
resource as trees can be grown in 5-10 yrs.
Give reasons:
Water is not used to control fires involving electrical equipment.
LPG is a better domestic fuel than wood.
Paper by itself catches fire easily whereas a piece of paper wrapped around an
aluminium pipe does not
a) Water is a good
conductor of electricity. If it is used for controlling a fire involving
electrical equipments, then the person dousing the fire
might get an electric
shock. Also, water can damage electrical equipments.
(b) LPG is a better domestic fuel as
it does not produce smoke and un-burnt carbon particles, which cause
respiratory problems.
(c) A piece of paper wrapped around
aluminium pipe does not catch fire easily. This is because
aluminium, being a
metal, is a good conductor of heat. Therefore, heat is transferred from the
paper to the metal and the paper does not attain its ignition temperature.
6) Make
a labelled diagram of a candle flame.
Name the unit in which the calorific value of a fuel is expressed.
ANSWER:-The calorific value of a fuel is
expressed in kilojoules per kilogram (kJ/kg).
Explain how CO2 is able to control fires.
ANSWER:-CO2, being heavier than oxygen, covers
the fire like a blanket. Since the contact between the fuel and oxygen is
cut off, the fire is
controlled. For fires involving electrical equipment and inflammable materials
like petrol, carbon dioxide (CO
2) is the best extinguisher.
The added advantage of CO
2 is that in most cases it does not harm the
electrical equipment. It not only forms a blanket around the fuel fire and cut
off oxygen supply, it also brings down the
temperature of the fuel. That is why it is an excellent fire extinguisher.
It is difficult to burn a heap of green leaves but dry leaves catch fire
ANSWER:-Green leaves have a lot of moisture in
them. This moisture does not allow them to catch fire easily. However, dry
leaves have no moisture in them. Therefore, they catch fire easily.
10) Which zone of a flame does a goldsmith
use for melting gold and silver and why?
ANSWER:-A goldsmith use outermost zone of the flame with a metallic
blow-pipe for melting gold and silver as it is the hottest part of flame.
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.
ANSWER:-The calorific value of fuel is the
amount of heat produced by the complete combustion of 1 kg of fuel.
Now, Heat produced by 4.5 kg of fuel =
180000 kJ
Calorific value of the fuel = Amount of heat energy
produced / Weight of Fuel burnt
= 180,000 /4.5 kJ/kg
= 40,000 kJ/J
Can the process of rusting be called combustion? Discuss.
ANSWER:-Combustion is a chemical process in
which a
substance reacts with oxygen and gives out energy during
the process in the form of either heat or
light or both. Rusting of iron is an exothermic process as heat is released
during rusting. Hence, it is a kind of slow combustion.
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?
ANSWER: The water in the Ramesh’s beaker will
heat up in
a shorter time. This is because the outermost zone of a
flame is the hottest zone, while the yellow
zone (in which Abida had kept the beaker) is less hot.