Exercises
Question 1:
List conditions under which combustion can take place.
Answer 1:
Conditions under which combustion can take place are as follows:
Fuel is required
Air (oxygen) is necessary.
Ignition temperature is required.
Question 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 __________.
Answer 2:
(a) Burning of wood and coal causes pollution of air.
(b) A liquid fuel, used in homes is Liquified 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.
Question 3:
Explain how the use of CNG in automobiles has reduced pollution in our cities.
Answer 3:
CNG produces very small amounts of harmful products like Sulphur dioxide, oxides of
nitrogen, oxides of sulphur etc. CNG is a cleaner fuel.
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Question 4:
Compare LPG and wood as fuels.
Answer 4:
Properties LPG Wood
1. Energy/unit mass Produces more energy Produces less energy
2. Pollution Less pollution More pollution
3. Residue Leave no residue Leave ash as residue
4. Storage/Transport Easy to store & transport Need more space to store
Question 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.
Answer 5:
(a) Water is not used to control fires involving electrical equipment because it may
conduct electricity and harm those trying to douse the fire.
(b) LPG is a good fuel as compared to wood as it readily available and it is cheap. It burns
easily in air at a moderate rate and it produces a large amount of heat. It does not
leave behind any undesirable substances.
(c) Paper by itself catches fire easily whereas a piece of paper wrapped around an
aluminium pipe does not because the ignition temperature of paper is low as
compare to the paper wrapped around an aluminium pipe.
Question 6:
Make a labelled diagram of a candle flame.
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Answer 6:
Question 7:
Name the unit in which the calorific value of a fuel is expressed.
Answer 7:
The calorific value of a fuel is expressed in a unit called kilojoule per kg (kJ/kg).
Question 8:
Explain how CO2 is able to control fires.
Answer 8:
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.
Question 9:
It is difficult to burn a heap of green leaves but dry leaves catch fire easily. Explain.
Answer 9:
The ignition temperature of green leaves is higher than that of the dry leaves, so dry
leaves catch fire easily.
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Question 10:
Which zone of a flame does a goldsmith use for melting gold and silver and why?
Answer 10:
Goldsmith uses the outermost zone of the flame for melting gold and silver because it is
the hottest zone (complete combustion) of the flame.
Question 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.
Answer 11:
𝐻𝑒𝑎𝑡 𝑃𝑟𝑜𝑑𝑢𝑐𝑒𝑑 (𝑖𝑛 𝑘𝑗)
𝐶𝑎𝑙𝑜𝑟𝑖𝑓𝑖𝑐 𝑉𝑎𝑙𝑢𝑒 =
𝑀𝑎𝑠𝑠 𝑜𝑓 𝐹𝑢𝑒𝑙 (𝑖𝑛 𝑘𝑔)
180,000 𝑘𝑗
=
4.5 𝑘𝑔
1800000
=
45
= 40000 𝑘𝑗/𝑘𝑔
Question 12:
Can the process of rusting be called combustion? Discuss.
Answer 12:
No, as rusting is a very slow process as compared to combustion and the heat evolve in
combustion is much more than rusting. Rusting can take place at room temperature but
combustion need an ignition temperature.
Question 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?
Answer 13:
The outermost part of the flame is the hottest one, so Rakesh’s water will get heated in a
shorter time.
