1.
List conditions under which combustion can take place.
Ans. Conditions
under which combustion can take place are as follows:
a.
Air
b.
The ignition temperature
c.
Inflammable substance.
2.
Fill in the blanks.
(a)
Burning of wood and coal causes ______________ of air.
(b)
A liquid fuel, used in home is ____________.
(c)
Fuel must be heated to its ________________ before it starts burning.
(d)
Fire produced by oil cannot be controlled by ____________.
Ans.
Fill
in the blanks.
(a) Burning of wood and
coal causes pollution
of air.
(b) A liquid fuel, used
in home is LPG.
(c) Fuel must be heated
to its ignition
temperature before it starts burning.
(d) Fire produced by oil
cannot be controlled by water.
3.
Explain how the use of CNG in automobiles has reduced pollution in our cities.
Ans. The
use of CNG in automobiles has reduced pollution in our cities because CNG does
not produce any poisonous gas on burning. That is why pollution in our cities
is reduced.
4.
Compare LPG and wood as fuels.
Ans. LPG
burns easily and produces more heat in comparison to wood. Besides, it is a
clean fuel, it does not produce fume and ashes as wood do. LPG can be stored
and transported easily and conveniently.
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 not used to control fire produced by electrical
equipment because water is conductor of electricity and may result in electric
shock.
(b)
LPG is a substance which is cheaper and burns easily. It produces large amount
of heat and does not leave behind any undesirable substance.
(c)
Paper catches fire easily, but when it is wrapped around an aluminium
pipe, the ignition temperature does not meet as heat is transferred to
aluminium to lower the temperature of paper.
6.
Make a labelled diagram of candle flame.
Ans. 
7.
Name the unit in which the calorific value of a fuel is expressed.
Ans. The calorific value of a fuel is expressed in kilojoules
per kg (kj/kg).
8.
Explain how CO2 is able to control fires.
Ans. Carbon
dioxide being heavier than oxygen covers the fire like a blanket. Since the
contact between fuel and oxygen is cut off, the fire is controlled. The added
advantage of carbon dioxide is that in most cases it does not harm the
electrical appliances.
9.
It is difficult to burn a heap of green leaves but dry leaves catch fire
easily. Explain.
Ans. Green
leaves contain water it cools the combustible materials, its temperature
is brought below its ignition temperature.
In dry leaves, water is absent so burning process start as the temperature is raised above the
ignition temperature and the leaves catch fire easily.
10.
Which zone of a flame does a goldsmith use for melting gold and silver and why?
Ans. The outermost zone of a flame , because it has the highest temperature to melt the
gold and silver.
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. Calorific
value of a fuel = Total heat produced/total mass burnt.
mass of fuel = 4.5 kg.
Heat produced = 180,000 kj.
Therefore, calorific value of fuel =
180,000/4.5kg = 40,000 kj/kg.
12.
Can the process of rusting be called combustion? Discuss.
Ans. Yes,
process of rusting can be called combustion, slow combustion, because
rusting also takes place in presence of air in the atmosphere.
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 of Ramesh’s beaker will get heated in a shorter time because the
outermost part of the flame is the hottest.
