Class 8 Cbse Ncert Science
Chapter 6, Combustion and Flame
Terms & Definitions
Calorific Value The
amount of heat energy produced on complete combustion of 1 kg of a fuel is
called its Calorific value, expressed in terms of kJ/kg.
Ignition Temperature It is the lowest temperature at which a combustible
substance can easily catch fire. Inflammable substances have very low ignition
temperature.
NCERT Answers of Textbook Exercise Questions (Important Only)
Question.1: List the conditions under which combustion can take place.
Answer: Following conditions are necessary
for combustion -
(i) The substance has to be combustible
(ii) It should have low ignition
temperature
(iii)
There
should be adequate supply of air.
Question.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 can not be controlled by _________.
Answer: (a) pollution (b) kerosene (c)
ignition temperature (d) water.
Question.3:
Explain how the use of CNG in automobiles has reduced pollution in our cities.
Answer: CNG has replaced patrol and diesel
as fuel in automobiles because these produce a lot of un-burnt carbon particles
and emit carbon dioxide and nitrogen oxides and sulphur dioxide. These all
gases are poisonous gases and cause various environmental hazards. But CNG is
safe because it produces these materials in very small amounts thus reducing
pollution in cities. Hence it becomes necessary to implement CNG in place of
other fuels in automobiles to reduce the pollution.
Question.4: Compare LPG and wood fuels.
Answer:
|
LPG
|
Wood
|
|
·
It
has more calorific value about 55000 kJ/kg
·
It
is smokeless fuel.
·
Easy
to transport.
·
Easily
stored in cylinders.
·
Does
not cause any environmental problem.
·
Have
very low ignition temperature.
|
·
It
has less calorific value about 17000 to 22000 kJ/kg
·
Gives
out lot of smoke which is quite dangerous.
·
Difficult
to transport.
·
Needs
a lot of space to store.
·
To
get it we need to trees to cut leading to deforestation which turn give rise
to many natural and environmental problems.
·
Ignition
temperature is not low.
|
Question.5: Give reasons:
a. Water
is not used to control the fire involving electrical equipment.
b. LPG
is better domestic fuel than wood.
c. Paper
by itself catches fire whereas a piece of paper wrapped around an aluminium
pipe does not.
Answer:
a) Water
is a conductor of electricity, so it can easily conduct electric current and
cause danger of electric shocks or short-circuits. Therefore water cannot be
used to control the fire involving electrical equipment.
b) There
are a number of reasons because of which we can say LPG is definitely a better
domestic fuel than wood. The reasons are - [Refer to the answer of Q.4 above].
c) Paper
by itself catches fire easily because it has low ignition temperature but when
wrapped around an aluminium pipe its temperature is lowered due to aluminium
metal absorbing the heat supplied to paper. So it does not catch fire.
Question.6: Make a labeled diagram of a candle flame.
Answer: Already given in Chapter 6, NCERT
Science textbook.
Question.9: It is difficult to burn a heap of green leaves but dry
leaves catch fire easily. Explain.
Answer: Green leaves have moisture in them,
which absorbs heat from them and raises their ignition temperature which does
not allow them to catch fire easily. On the other hand dry leaves have low
ignition temperature and so, burn easily.
Question.10: Which zone of a flame does a goldsmith use for melting
gold and silver and why?
Answer: A goldsmith uses the outer part of
the candle flame for melting gold and silver because in this zone the
temperature is the highest which helps to melt these metals easily.
Question.11: In an experiment 4.5 kg of a fuel was completely burnt.
The heat produced was measured to be 180000 kJ. Calculate the calorific value
of the fuel.
Answer: Heat produced after burning 4.5 kg
fuel = 180000 kJ
Or, Heat produced after
burning 1 kg fuel = (180000 ÷ 4.5) kJ = 40000kJ
So, Calorific value of
fuel = 40000 kJ/kg
(Please be in touch of this site CBSE Guide NCERT Solutions : Soon we
will be posting additional important CBSE Sample Questions on this Chapter,
Combustion and Flame)
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