NCERT Solutions Std 8 Science Combustion and Flame

Combustion and Flame


1). List conditions under which combustion can take place.

Ans: The conditions under which combustion can take place are as follows.

i). presence of a combustible substance

ii). Presence of oxygen in the air.

iii). Attaining ignition temperature.

2). Fill in the blanks.

(a) Burning of wood and coal causes pollution of air.

(b) A liquid fuel, used in homes is kerosene.

(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 CNG has reduced pollution in our cities in the following ways.

i). It produces harmful products like carbon monoxide and carbon dioxide in very small amounts.

ii). It is a cleaner fuel.

iii). No residue remains after combustion.

4). Compare LPG and wood as fuels.

LPG Wood
1. LPG is a gaseous fuel. 1. Wood is a solid fuel.
2. It does not produce smoke. 2. It produces smoke.
3. It is easily stored in cylinders. 3. It requires more space to store
4. It does not cause any pollution. 4. It causes pollution.

5). Give reasons.

(a) Water is not used to control fires involving electrical equipment.

Ans: If elelctical equipment is on fire, water may conduct electricity and harm those trying to douse the fire. Therefore, water is not used to control fires involving electrical equipment.

(b) LPG is a better domestic fuel than wood.

Ans: LPG have more calorific value than wood and it causes less pollution. Therefore, it 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 combustible substance cannot catch fire or burn as long as its temperature is lower than its ignition temperature. The ignition temperature of paper is less therefore it catches fire easily. It does not catch fire when it is wrapped around an aluminium pipe because aluminium absorbs the heat and paper does not attain its ignition temperature.

6). Make a labeled diagram of a candle flame.


7). Name the unit in which the calorific value of a fuel is expressed.

Ans: The calorific value of a fuel is expressed in Kilojoule per kg.

8). Explain how CO2 is able to control fires.

Ans: 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.

9). It is difficult to burn a heap of green leaves but dry leaves catch fire easily. Explain.

Ans: Green leaves contains some amount of water due to which ignition temperature increases and they do not catch the fire easily. Dry leaves has no water so they catch fire easily.

10). Which zone of a flame does a goldsmith use for melting gold and silver and why?

Ans: A goldsmith uses the outer zone of a candle flame to melt gold and silver because it is the hottest zone and has more temperature.

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: Mass of the fuel = 4.5 kg

Heat produced = 180,000 kJ

Heat produced for 1 kg of fuel = 180000/4.5

                                               = 180000/4500

                                               = 40000 kJ/kg.

12). Can the process of rusting be called combustion? Discuss.

Ans:The process of rusting cannot be called combustion because no light and heat is produced in this reaction.

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: Outermost part of the flame has high temperature. Therefore, the water heated by the Ramesh will get heated in a shorter time.

Additional Questions

Fill in the blanks.

i). For combustion air is necessary.

ii). The substances that gives heat and light after combustion are called fuel.

iii). Coal burns in air producing carbon dioxide, heat and light.

iv). The most common fire extinguisher is water.

v). For fire involving electrical equipment and inflammable materials like petrol, carbon dioxide is the best extinguisher.

vi). The calorific value is expressed in a unit called Kilojoule per kg.

vii). Fuels differ in the efficiency and cost.

Write whether True or False.

i). Magnesium burns to form magnesium oxide.

Ans: True.

ii). The fuel may be solid, liquid, and gas.

Ans: True.

iii). Food is fuel for the body.

Ans:  True.

iv). The most common fire extinguisher is water.

Ans: True.

v). Petrol is an ideal fuel.

Ans:  False

vi). The substance which have low ignition temperature will not catch fire.

Ans: False

vii). CNG and LPG are the examples of liquid fuels.

Ans: False

viii). Combustion is a physical process

Ans: False

ix). All the fuels burn with a flame.

Ans: False


i). Combustion

Ans: A chemical process in which a substance reacts with oxygen to give off heat is called combustion.

ii). Ignition Temperature

Ans: The lowest temperature at which a substance catches fire is called its ignition temperature.

iii). Inflammable substances

Ans: The substances which have very low ignition temperature and can easily catch fire with a flame are called inflammable substances.

iv). Calorific value

Ans: The amount of  heat energy produced on complete combustion of 1 Kg of a fuel is called its calorific value.

Answer in one sentence.

1). What is meant by combustible substance?

Ans: The substance that undergoes combustion is called combustible.

2). Write two examples of combustible substances.

Ans: Charcoal and magnesium are combustible substances.

3). What are inflammable substances?

Ans: The substances which have very low ignition temperatures and can easily catch fire with a flame are called inflammable substances.

4). Give examples of inflammable substances.

Ans: Examples of inflammable substances are petrol, alcohol, and Liquified Petroleum Gas (LPG).

5). What are fules?

Ans: The substances that provide heat and light are called fuels.

6). Which is the most common fire extinguisher?

Ans: Water is the most common fire extinguisher.

Answer in short.

1). Explain how fire is extinguished by a fire brigade?

Ans: The fire brigade pours water on the fire. Water cools the combustible material so that its temperature is brought below its ignition temperature. This prevents the fire from spreading. Water vapors also surround the combustible material, helping in cutting off the supply of air. So, the fire is extinguished.

2). Explain in brief the types of combustion.

Ans: The types of combustion are as follows.

Rapid combustion: The combustion that burns rapidly and produces heat and light is known as rapid combustion.

Spontaneous combustion: The type of combustion in which a material suddenly bursts into flames without the application of any apparent cause is called spontaneous combustion.

3). Write the characteristics of good fuel?

Ans: The characteristics of a good fuel are as follows.

i). It is readily available.

ii). It is cheap.

iii). It burns easily in air at a moderate temperature.

iv) It produces large amount of heat.

v). It does not leave behind any undesirable substances.

4). Write the names of different zones of the flame?

Ans: The three different zones of flames are

i). Non-luminous zone

ii). Luminous zone

iii). Dark zone.

5). What is global warming?

Ans: Combustion of most fuels releases carbon dioxide in the environment. Increased concentration of carbon dioxide in the air is believed to cause global warming.

6). What is acid rain?

Ans: Burning of coal and diesel releases sulphur dioxide gas. It is an extremely suffocation and corrosive gas. Moreover, petrol engines give off gaseous oxides of nitrogen. Oxides of sulphur and nitrogen dissolve in rain water and form acids. Such rain is called acid rain. It is very harmful for crops, buildings and soil.

Classify the following as combustible and non-combustible substances.

Wood, paper, iron, kerosene, stone, straw, charcoal, matchsticks, glass.

Combustible Substances Non-combustible Substances












Classify the following as solid, liquid and gaseous fuels.

Wood, kerosene, coal, natural gas, petrol, LPG, gobar gas, cow-dung, diesel.

Solid Fuels Liquid Fuels Gaseous  Fuels









Natural gas


Gobar gas




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