COMBUSTION AND FLAME CLASS VIII SCIENCE CHAPTER 6 NCERT/CBSC SOLUTIONS WITH SUMMARY OF THE CHAPTER

 SCIENCE SAMADHAN  

                                     PRIYANKA PANDEY

NCERT SOLUTIONS FOR CLASS-VIII SCIENCE

CHAPTER-6

                         COMBUSTION AND FLAME

                                

                 

                   SUMMARY OF THE CHAPTER

Combustion

• A chemical process in which a substance reacts with oxygen to give off heat and light is called combustion.

• The burning of wood is an example of combustion

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Combustible and Non-Combustible Substances

• Substances, which easily catch fire are combustible substances,

    such as paper, coal, wood.

• Substances, which do not catch fire readily are non-combustible substances, such as sand, water, glass.

Fuel

• Any substance, which upon combustion produces a usable amount of energy is known as fuel. For example, fossil fuels, biogas, nuclear energy etc.

• Fuels can be solid, liquid or gas depending on their state.

• BIOGAS PLANT

• NUCLEAR POWER PLANT

Ignition Temp

The lowest temperature at which a combustible substance catches fire when heated in the air is called its ignition temperature.

Inflammable Substances

The substances, which have very low ignition temperature and can easily catch fire with a flame are known as inflammable substances. Examples: diesel, LPG, acetone.

Fire

• Fire is the result of a chemical combustion reaction between oxygen and some sort of fuel.​​​​​​

• How long a fire lasts depends on how much fuel and oxygen are available.

Flame

• Flame is the visible and gaseous part of the fire.

• What we see as the flame is the light energy released due to the combustion of fuel.

Zones of Candle Flame

Structure of Flame

• The outermost zone is the hottest among all zones and is blue in colour, and this is due to complete combustion. It is the non-luminous part of the flame.

• The middle zone of the candle flame is moderately hot and is yellow in colour, and partial combustion of fuel takes place. It is the bright part of the flame.

• The innermost zone of the flame is the least hot and is black in colour. This is due to the presence of unburnt wax vapours.

Smoke

• Smoke is an example of solid (unburnt particles) dispersed in a gas (air).

• The black colour of smoke is due to the presence of unburnt carbon particles in the smoke.

Types of Combustion

       1) Rapid combustion

• The type of combustion in which heat and light are released in a very short span of time is called rapid combustion. For e.g. combustion of L.P.G

2)Spontaneous combustion

• The type of combustion in which substances catch fire on their own, without the application of heat is termed as spontaneous combustion. For e.g. forest fires, fire in coal mines.

Working of a Matchstick

• The main component of the bulb of a matchstick is red phosphorus which turns into white phosphorus on heating.

• White phosphorus spontaneously ignites, thereby increasing the temperature of the wooden stem to the ignition point and the matchstick starts burning.

Fire Control

Fire can be controlled by removing any or all of the factors of combustion, i.e. fuel, oxygen (air) and ignition temperature (by lowering the temperature).

Fire Extinguisher

• The fire extinguisher is a device used by the fire brigade to control fire.

• The role of the fire extinguishers is to cut off the supply of oxygen or bring down the temperature of the fuel or both.

Ideal Fuel

• The ideal fuel is cheap, easily available and readily combustible.

• It has high calorific value.

• It does not produce harmful gases or residues that pollute the environment.

Calorific Value and Efficiency of a Fuel

• The amount of heat energy produced on complete combustion of 1 kg of a fuel is called its calorific value. The calorific value of a fuel is expressed in a unit called kilojoule per kg (kJ/kg).

• Efficiency is that proportion of energy released by a fuel combustion process which is converted into useful work.

• Calorific value is directly proportional to its efficiency. If the value is high, it’s efficiency will also be high. If the value is low, it’s efficiency would also be low.

Pollution

Harmful Products from Burning of Fuel

• The burning of fuels like wood, coal and petroleum products releases unburnt carbon particles in the air which causes respiratory problems.

• Incomplete combustion of fuels produces a very poisonous gas called carbon monoxide.

• The burning of fuels releases carbon dioxide in air in the environment which causes global warming.

Unburnt Carbon Particles

• Carbon fuels like wood, coal, candle,  petroleum release unburnt carbon particles.

• These fine particles are dangerous pollutants causing respiratory diseases, such as asthma

                                        

 EMISSION OF CARBON MONOXIDE 

• Carbon monoxide is a poisonous gas, which is produced by incomplete combustion of fuels.

• It is dangerous to burn coal in a closed room as the carbon monoxide produced can kill persons sleeping in that room.

Global Warming

• The rise in the average temperature of the earth’s atmosphere due to the release of carbon dioxide on combustion of fuels is termed as global warming.

• Melting of polar ice-caps or change in the rainfall pattern are the consequences of global warming.

Acid Rain

• Acid rains are caused by emissions of sulphur dioxide and nitrogen oxide, which react with the water molecules in the atmosphere to produce acid.

• It has a very harmful effect on plants, land and aquatic animals and infrastructure.

CNG – The Clean Fuel

The use of diesel and petrol as fuels in automobiles is being replaced by CNG (Compressed Natural Gas) because it is less polluting and a cleaner fuel.

 NCERT Textbook Questions

Question 1.
List conditions under which combustion can take place.
Answer:
Combustion can take place in the presence of:
(a) a combustible substance.
(b) oxygen, that is, the supporter of combustion.
(c) attainment of ignition temperature of the substance.

Question 2.
Fill in the blanks.
(a) Burning of wood and coal causes pollution of air.
(b) A liquid fuel, used in homes 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.

Question 3.
Explain how the use of CNG in automobiles has reduced pollution in our cities.
Answer:
The use of CNG in automobiles has reduced pollution in our cities as it is a quality fuel and has some benefits:
(a) It gives out less carbon dioxide gas, carbon monoxide gas, sulphur dioxide and nitrogen dioxide, which is beneficial as they play crucial role in global warming and acid rain.
(b) It leaves behind no residue after its combustion.

Question 4.
Compare LPG and wood as fuels.
Ans

LPG	Wood
(i) It does not cause pollution on combustion.	(i) It cause air  pollution on combustion.
(ii) No smoke is produced.	(ii) It produces smoke.
(iii) It is a liquid fuel.	(iii) It is a solid fuel.
(iv) It has more calorific value (55000 kJ/kg).	(iv) It has less calorific value (17000 kJ/kg).
(v) It can be easily transported, as it is stored in cylinders.	(v) It can’t be transported easily like LPG fuels.

 

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:
(a) Since water is a good conductor of electricity, it may result in electric shocks to the person trying to extinguish fire.
(b) LPG is better domestic fuel than wood because it does not produce gases, nor does it leave any residue behind. Moreover, it has more calorific value than wood.
(c) As its ignition temperature is low, the paper by itself catches fire easily. But a piece of paper wrapped around an aluminium pipe does not catch fire easily, as the heat being given gets absorbed by the aluminium pipe and the piece of paper does not get its ignition temperature.

Question 6.
Make a labelled diagram of a candle flame.
Answer:

Question 7.
Name the unit in which the calorific value of a fuel is expressed.
Answer:
The unit in which the calorific value of a fuel is expressed is kilojoules per kilogram (kJ/kg).

Question 8.
Explain how CO2 is able to control fires.
Answer:
As CO2 is heavier than oxygen, it forms a blanket around fire, because of which the supply of air is stopped. It also brings down the temperature of the burning substance. In these ways, it plays a significant role in controlling fire.

Question 9.
It is difficult to burn a heap of green leaves but dry leaves catch fire easily. Explain.
Answer:
The green leaves hold some amount of water, so its ignition temperature gets increased and it does not burn easily. On the other hand, dry leaves are waterless, so they catch fire easily (having low ignition temperature).

Question 10.
Which zone of a flame does a goldsmith use for melting gold and silver and why?
Answer:
A goldsmith uses the outermost zone of a flame, which is non-luminous, to melt gold and silver as it is the hottest zone of the flame, having more temperature.

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:

Total mass of fuel=4.5kg

Total heat produced=180,000kJ
Calorific value of a fuel =   Heat Produced / Amount of fuel
180000 / 4.5 kJ/kg
= 40,000 kJ/kg.

Question 12.

Can the process of rusting be called combustion? Discuss.
Answer:
The process of rusting emits heat during the formation of its oxide. So we can call the process of rusting as slow combustion.

 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:
The water which was put by Ramesh will get heated in a shorter time; because he had put it nearer to the hottest zone of the flame.

                                 ADDITIONAL QUESTIONS

Question 1.
What are the main constituent of biogas and kitchen gas (L.P.G.) ?
Answer:
The main constituent of biogas is methane and of kitchen gas (L.P.G.) is butane

Question 2.
What is biogas ?
Answer:
Biogas is formed by the decomposition of plant and animal wastes.

Question 3.
List conditions under which combustion can take place.
Answer:
The conditions under which combustion can take place :

• Presence of combustible substance.

• Presence of supporter of combustion i.e., oxygen.

• Attainment of ignition temperatue

Question 4.
What is meant by global warming ?
Answer:
Global warming is the rise in temperature of the environment of the earth.

Question 5.
What is acid rain ?
Answer:
Sulphur dioxide and nitrogen oxides dissolve in rain water and form acids. Such rain is called acid rain.

Question 6.
How is acid rain harmful ?

Answer:
Acid rain corrodes buildings and reduces the fertility of soil by making acidic.

Question 7.
Why is a smelling agent added to LPG ?
Answer:
LPG does not have any smell, so to detect the leakage of the gas, a smelling agent is added to it.

Question 8.
A person sleeping in a closed room with burning coal, feels suffocated after sometime. Why ?
Answer:
In a closed room, carbon monoxide is produced which is a poisonous gas.

Question 9
Write any four characteristics of an ideal fuel. What is C.N.G. ? Mention its one use.
Answer:

• It is cheap and easily available. ,

• It does not produce any harmful gas during burning.

• It has high calorific value.

• It is safe and easy to store.

C.N.G – Compressed Natural Gas. It is used as a fuel for automobiles.

Question 10.
When the clothes of a person catches fire, we cover him with a blanket. Why ?
Answer:
When the person is covered with a blanket, the supply of oxygen is cut off. So, the fire is put off. 

                       LEARN &  SHINE   

                                                    PRIYANKA PANDEY

•            Priyanka Pandey

              SCIENCE SAMADHAN