NCERT Class 8 Science Chapter 4 ‘Combustion and Flame’: Notes and Solutions (Free PDF)

Days leading up to exams are surely stressful times for students. During these stressful times, we realise the importance of summarised notes. Hence, to help you in these testimes, we have prepared NCERT Solutions and Notes Class 8 Science Chapter 4: Combustion and Flame for you. In these notes, we have covered every topic concisely and accurately. Thereafter, in the notes, you will find Solutions to the NCERT exercises and a PDF of the same is also available for you to download. Now, let’s explore this blog! 

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Chapter 4	Chapter 5	Chapter 6
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Class 8 Science Chapter 4: Combustion and Flame Notes

Let’s explore the notes of Class 8 Science Chapter 4: Combustion and Flame. These notes have been prepared keeping in mind to cover all the important aspects of the chapter in a concise manner and in simple language making it easier for you to understand them.

Also Read: Science Projects for Class 8

Combustion

The process in which a substance reacts with the oxygen in the air to produce heat is known as combustion. The substance that undergoes combustion is known as a combustible substance.

Different substances burn at different temperatures. The lowest temperature at which a substance catches fire is known as its ignition temperature. As long as the temperature of a substance is lower than its ignition temperature, it cannot burn.

Substances that have a very low ignition temperature and catch fire very easily are known as inflammable substances. Example:- petrol, LPG, etc.

How Do We Control Fire?

For a fire to start, there are 3 major requirements:- fuel, air supply, and heat. Even if any one of these conditions is not meant or cut off, fire will be extinguished. Let’s look at how water and fire extinguisher extinguish a fire:-

1. Water:- 

• It extinguishes a fire by cooling the substance that is on fire. It brings the temperature of the fuel or the combustible substance below its ignition temperature. The water vapours formed surround the substance and cut off the air supply. Hence, the fire gets extinguished.
• Although, water can only be used to extinguish fires that involve wood or paper, etc, if any electrical instrument is involved, then it can harm the people trying to extinguish the fire

2. Fire-Extinguisher:-

• Fire extinguishers produce C02. In cases of fires in electrical equipment, or oil, or petrol, etc, they are the best way to extinguish the fire.
• CO2 in them forms a blanket around the combustible substances and cuts off their supply of air thus extinguishing the fire.

Types of Combustion

There are basically 3 types of combustion:-

1. Rapid Combustion:- The type of combustion in the case of gases that produces heat and light and is rapid is known as rapid combustion.
2. Spontaneous Combustion:- The combustion in which a substance suddenly catches flame without any apparent cause is known as spontaneous combustion.
3. Explosion:- The combustion in which there is a sudden release of heat, light, and sound along with the release of a large amount of gas is known as an explosion. 

Also Read: List of NCERT Books Class 8

Class 8 Science Chapter 4: Combustion and Flame Notes: Flame

Flame is formed by the substance that vaporises on burning. For example, kerosene oil and molten wax rise through the wick and are vapourised during burning and form flames.

Different chemicals, on burning, give different coloured flames.

What is a Fuel?

The substances that can be burnt to produce heat energy are known as fuel. 

Characteristics of an ideal fuel are:-

• It is readily available.
• It is reasonable in terms of cost.
• It burns easily in the air at a moderate rate.
• It produces a large amount of heat.
• It does not leave behind any undesirable substances.

Fuel Efficiency

The amount of heat energy that is produced when 1 kg of fuel undergoes complete combustion is known as the calorific value. It is expressed in the unit “Kilojoule per kg” (kJ/kg).

Burning of Fuels Leads to Harmful Products

The burning of fuels has harmful effects on the environment. Those effects are mentioned below:- 

• Carbon fuels like wood, coal, and petroleum release unburnt carbon particles that are dangerous pollutants and can cause respiratory diseases such as asthma, etc. 
• The incomplete combustion of these fuels gives carbon monoxide gas that is poisonous and can kill a person. It is produced when coal, etc is burnt in a closed room. 
• Combustion of most fuels releases carbon dioxide in the environment which, in turn, causes global warming.
• Burning of coal and diesel releases sulphur dioxide gas which is an extremely suffocating and corrosive gas. Petrol engines give off gaseous oxides of nitrogen. Oxides of sulphur and nitrogen dissolve in rainwater and form acids. Such rain is called acid rain. It is very harmful to crops, buildings and soil. 

Because of such reasons, petrol and diesel are being replaced with CNG which produces significantly less harmful products that too in small amounts. CNG is a much cleaner fuel. 

Important Definitions in NCERT Class 8 Chapter 4 Combustion and Flame

Some important definitions from the chapter are mentioned below:

• Combustion: The process in which a substance reacts with the oxygen in the air to produce heat is known as combustion.
• Combustible Substance: The material or object that undergoes combustion is known as a combustible substance.
• Ignition Temperature: The lowest temperature at which a substance catches fire is known as its ignition temperature.
• Inflammable Substances: Materials that have a very low ignition temperature and catch fire very easily are known as inflammable substances.
• Rapid Combustion: The type of combustion in the case of gases that produces heat and light and is rapid is known as rapid combustion.
• Spontaneous Combustion:-The combustion in which a substance suddenly catches flame without any apparent cause is known as spontaneous combustion.
• Explosion: The combustion in which there is a sudden release of heat, light, and sound along with the release of a large amount of gas is known as an explosion. 
• Fuel: The substances that can be burnt to produce heat energy are known as fuel. 
• Calorific Value: The amount of heat energy that is produced when 1 kg of fuel undergoes complete combustion is known as the calorific value. It is expressed in the unit “Kilojoule per kg” (kJ/kg).
• Acid Rain: Oxides of sulphur and nitrogen dissolve in rainwater and form acids. Such rain is called acid rain. It is very harmful to crops, buildings and soil. 

Explore NCERT Solutions of all Chapters of Class 8 Science:

Chapter 1	Chapter 2	Chapter 3
Chapter 4	Chapter 5	Chapter 6
Chapter 7	Chapter 8	Chapter 9
Chapter 10	Chapter 11	Chapter 12
Chapter 13

Download the NCERT Class 8 Science Chapter 4 Solutions PDF Here!

NCERT Solutions of Class 8 Science Chapter 4 Combustion and Flame

Let us now explore NCERT Solutions of Class 8 Science Chapter 4 to get answers to all writing questions in the lesson Combustion and Flame.

Exercise Questions

1. List conditions under which combustion can take place.

Soln:

For combustion to start, there are 3 major requirements:- fuel, air supply, and heat. Even if any one of these conditions is not meant or cut off, fire will be extinguished. 

2. 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) The fire produced by oil cannot be controlled by___________ .

Soln:

(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) The fire produced by oil cannot be controlled by water.

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

Soln:

The role of CNG in reducing pollution is quite significant because of the following reasons:- 

• The combustion of fuels like petroleum causes many unburnt carbon particles along with carbon monoxide, which leads to respiratory diseases. Along with this, they also contribute to pollution. 
• CNG is a much cleaner fuel.
• The CNG can be an alternative for diesel, petrol and propane/LPG.
• It usually contains a few undesirable gases than the other fuels mentioned above.
• CNG produces significantly less harmful products, that too in small amounts.

4. Compare LPG and wood as fuels.

Soln:

Wood

• It is a traditional fuel that is used for both domestic and industrial purposes.
• It produces a lot of smoke that pollutes the atmosphere and causes respiratory diseases such as asthma.
• Using wood as a fuel is leading to deforestation. 
• The calorific value of wood ranges between 17000 and 22000 kJ/kg
• However, wood may be used as a furnace, stove or fireplace indoors while it is used for a campfire, or furnace outdoors.

LPG

• The usage of LPG (Liquefied petroleum gas) has replaced wood.
• LPG does not release smoke and other pollutants.
• It is a much cleaner fuel.
• The fuel efficiency of LPG is more than that of wood.
• The calorific value of LPG is 55000 kJ/kg.
• LPG is now the most preferred choice.

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.

Soln:

a)

• Water can only be used to extinguish fires that involve wood or paper, etc.
• If any electrical instrument is involved, then it can harm the people trying to extinguish the fire
• Because of its electricity-conducting properties, it cannot be used to extinguish electrical equipment fires. 

b)

• LPG is a much cleaner fuel than wood and doesn’t release smoke and other pollutants.
• Wood leads to the release of a lot of smoke and fumes polluting the atmosphere causing pollution and leading to respiratory diseases.
• Therefore, LPG is a better domestic fuel than wood.

c)

• The ignition temperature of paper is low, hence it easily catches fire.
• But paper wrapped around an aluminium pipe does not catch fire easily because aluminium is a good conductor of heat. This results in the transferring of heat from paper to aluminium which has a very high ignition temperature.
• Hence, the paper does not catch fire in such case.

6. Make a labelled diagram of a candle flame.

Soln:

Credits: Byjus

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

Soln:

The amount of heat energy that is produced when 1 kg of fuel undergoes complete combustion is known as the calorific value. It is expressed in the unit “Kilojoule per kg” (kJ/kg).

8. Explain how CO2 is able to control fires.

Soln:

CO2 is a non-combustible gas and extinguishes fire in two ways:

(i) CO2 is heavier than oxygen, covers the fire like a blanket and thus it cuts off the supply of air. 

(ii) In cylinders, CO2 is kept in the liquid form. When released, it expands enormously. This brings down the temperature of the fuel, which helps in controlling the fire.

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

Soln:

A heap of green leaves has a lot of water in its elements such as veins, cuticles, etc making its ignition temperature high. Therefore, it does not catch fire easily.

But dry leaves have no moisture content in them, hence their ignition temperature is low. Therefore, they catch fire easily.

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

Soln:

A goldsmith mainly uses the non-luminous zone which is the outermost zone of flame. This part of the flame is used because the outermost flame undergoes complete combustion and is considered to be the hottest part of the flame.

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.

Soln:

Credits: Byjus

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

Soln:

No, rusting cannot be called combustion because it is an exothermic process. Combustion is the process in which a substance reacts with the oxygen in the air to produce heat.

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?

Soln:

Ramesh’s water will get heated in a shorter time as it is placed in the outermost part of the flame which is a non-luminous zone and is considered the hottest part of the flame. On the other hand, Abida’s beaker is placed in the luminous part of the flame that is less hot, hence it will take more time to heat. 

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