NCERT Solutions for Class 8 Science Chapter 4 Combustion and Flame

NCERT
SOLUTIONS
CLASS - 8TH

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Book : Science Ncert Solutions | Chapter-6 Science

Class : 8th
Subject : Science
Chapter : 6
Chapter Name : COMBUSTION AND FLAME

Q1 List conditions under which combustion can take place.

Answer. Combustion is a process of reaction of a substance with oxygen. 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 �re)

Q2 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. (a) Burning of wood and coal causes pollution of air.
(b) A liquid fuel used in homes is lique�ed 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

Q3 Explain how the use of CNG in automobiles has reduced pollution in our cities

Answer. Combustion of fuels like petroleum causes formation of unburnt 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.

Q4 Compare LPG and wood as fuels.

Answer. Wood has been a traditional fuel for both domestic and industrial use. However, it produces a lot of
smoke that can cause respiratory problems. Also, wood is obtained from trees. Thus, using wood as a fuel
causes deforestation. Therefore, slowly wood is replaced by LPG, which is a lique�ed form of petroleum gas.
It does not give out smoke and other pollutants and is a cleaner fuel. Again, fuel ef�ciency of LPG is more
than that of wood. The calori�c value of LPG is 55000 kJ /kg, while that of wood is between 17000 to 22000
kJ / kg. Hence, LPG is favoured over wood.

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Book : Science Ncert Solutions | Chapter-6 Science

Q5 Give reasons.
(a) Water is not used to control �res involving electrical equipment.
(b) LPG is a better domestic fuel than wood.
(c) Paper by itself catches �re easily whereas a piece of paper wrapped around an aluminium pipe does not.

Answer. (a) Water is a good conductor of electricity. If it is used for controlling a �re involving electrical
equipments, then the person dousing the �re 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 unburnt carbon particles, which cause
respiratory problems.
(c) A piece of paper wrapped around aluminium pipe does not catch �re 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.

Q6 Make a labelled diagram of a candle �ame.

Q7 Name the unit in which the calori�c value of a fuel is expressed

Answer. The calori�c value of a fuel is expressed in kilojoules per kilogram (kJ/kg).

Q8 Explain how CO2 is able to control �res

Answer. C02 is a non-combustible gas and extinguishes �re in two ways:
(i) Since it is heavier than oxygen, it covers the �re like a blanket and cuts off the contact
between oxygen and fuel.
(ii) In cylinders, C02 is kept in the liquid form. When released, it expands enormously and
s down. This brings down the temperature of the fuel, which helps in controlling the �re.

Q9 It is dif�cult to burn a heap of green leaves but dry leaves catch �re easily. Explain

Answer. Green leaves have a lot of moisture in them. This moisture does not allow them to catch �re easily.

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Book : Science Ncert Solutions | Chapter-6 Science

However, dry leaves have no moisture in them. Therefore, they catch �re easily.

Q10 Which zone of a �ame does a goldsmith use for melting gold and silver and why?

Answer. Goldsmiths use the outermost part/zone of the �ame to melt gold and silver. This is because the
outermost zone of the �ame undergoes complete combustion and is the hottest part of the �ame.

Q11 In an experiment 4.5 kg of a fuel was completely burnt. The heat produced was measured to be 180,000
kJ. Calculate the calori�c value of the fuel.

Q12 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.

Q13 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 �ame. Ramesh kept the beaker in the outermost
part of the �ame. 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 �ame is the hottest zone, while the yellow zone (in which Abida had kept the beaker) is less hot.

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