Chapter 6: Combustion and flame

These NCERT Class 8 Science Chapter 6 notes explain combustion and the structure of flame in a simple and structured manner. The chapter focuses on how substances burn and produce heat and light.

It discusses types of combustion such as rapid combustion, spontaneous combustion and explosion. It also explains ignition temperature, inflammable substances, fire triangle and zones of a flame.

Chapter 6 – Combustion and Flame is important for understanding fuels, energy production and fire safety. It is highly relevant for school examinations and foundation level competitive preparation.

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1. Introduction: what is combustion?

• Combustion is a chemical process in which a substance reacts with oxygen.
• During combustion, heat is always produced.
• Often light is also produced along with heat.
• Burning of LPG in the kitchen is an example of combustion.
• Burning of coal in a stove is another example.

2. What is a fuel?

• A fuel is a substance that produces useful energy on burning.
• A good fuel produces a large amount of heat and has high calorific value.
• It should be easily available, cheap and easy to store and transport.
• It should produce less smoke and harmful gases.
• Examples include wood, coal, petrol, LPG and CNG.

3. Types of combustion

• Rapid combustion is burning that happens quickly and produces heat and light immediately, like LPG burning.
• Spontaneous combustion occurs when a substance catches fire on its own without external flame.
• White phosphorus catching fire in air is an example of spontaneous combustion.
• Explosion is very fast combustion producing large amounts of heat, light, sound and gas.
• Fire crackers and bomb explosions are examples of explosion.

4. Conditions necessary for combustion

• Combustion requires three things: fuel, oxygen and heat.
• These three components together form the fire triangle.
• Removal of any one component stops the fire.
• Water lowers the temperature and stops combustion.
• Sand cuts off oxygen supply and extinguishes fire.

5. Ignition temperature and inflammable substances

• Ignition temperature is the lowest temperature at which a substance catches fire.
• Petrol has a low ignition temperature and catches fire easily.
• Wood has a higher ignition temperature.
• Inflammable substances are substances with very low ignition temperature.
• Examples of inflammable substances include petrol, LPG and alcohol.

6. Flame and its formation

• A flame is the visible, glowing part of fire.
• Only substances that vaporise on burning produce a flame.
• Candle flame and LPG flame are common examples.
• Some substances like charcoal burn without flame.
• Flame formation depends on the presence of vapours.

7. Structure of a flame

• A flame has three zones: inner zone, middle zone and outer zone.
• The innermost zone is dark, least hot and contains unburnt fuel.
• The middle zone is yellow, moderately hot and shows partial combustion.
• The outer zone is blue, hottest and shows complete combustion.
• The yellow part of flame is called luminous flame, while blue part is non-luminous flame.

8. Types of flame

• A luminous flame is yellow in colour and produces smoke.
• Luminous flame is less hot and seen in candle burning.
• A non-luminous flame is blue in colour and very hot.
• Non-luminous flame does not produce smoke.
• LPG stove flame is an example of non-luminous flame.

9. Fuel efficiency and calorific value

• Calorific value is the amount of heat produced by burning 1 kg of fuel.
• It is measured in kilojoule per kilogram (kJ/kg).
• Fuels with higher calorific value produce more energy.
• LPG has higher calorific value than coal and wood.
• Cow dung cake has lower calorific value compared to other fuels.

10. Harmful effects of burning fuels

• Burning fuels causes air pollution.
• It releases harmful gases like carbon monoxide and carbon dioxide.
• Excess carbon dioxide leads to global warming.
• Some gases cause acid rain.
• Incomplete combustion produces toxic gases.

11. Ideal fuel for the future

• An ideal fuel should be clean and produce less pollution.
• It should be cheap and easily available.
• It should have high calorific value.
• CNG and biogas are considered better fuels.
• Solar energy is a clean alternative source of energy.

12. Fire control and fire extinguishers

• Fire can be controlled by removing fuel.
• Cutting off oxygen supply also stops fire.
• Lowering temperature below ignition temperature extinguishes fire.
• Fire extinguishers use water, foam or carbon dioxide.
• Carbon dioxide extinguisher is used for electrical fires as it does not conduct electricity.

13. Key exam-oriented keywords explained

• Combustion: Chemical process in which a substance reacts with oxygen producing heat and light.
• Fuel: Substance that produces useful energy on burning.
• Ignition temperature: Lowest temperature at which a substance catches fire.
• Inflammable substances: Substances that catch fire easily due to low ignition temperature.
• Rapid combustion: Quick burning producing immediate heat and light.
• Spontaneous combustion: Self-ignition of a substance without external flame.
• Explosion: Sudden and fast combustion producing heat, light and sound.
• Flame: Visible glowing part of fire.
• Luminous flame: Yellow flame that produces smoke and is less hot.
• Non-luminous flame: Blue flame that is hotter and smokeless.
• Calorific value: Amount of heat produced by burning 1 kg of fuel.
• Fire triangle: Combination of fuel, oxygen and heat required for combustion.

14. One-page summary for quick revision

• Combustion is the burning of a substance in presence of oxygen.
• Fuel produces energy and has a specific calorific value.
• Fire requires fuel, oxygen and heat forming the fire triangle.
• A flame has three zones and outer zone is the hottest.
• Burning fuels causes pollution, so cleaner fuels should be used.

Understanding NCERT Class 8 Science Chapter 6 is essential to learn how fuels burn and how energy is produced. The chapter clearly explains calorific value, flame structure and harmful effects of burning fuels.

The concepts help students understand air pollution, fire control and efficient fuel use. It is highly useful for school exams and foundation competitive preparation.

Continue reading NCERT Class 8 Science Chapter 7 – Conservation of Plants and Animals to understand biodiversity and environmental protection.

FAQs

Q1. What is combustion?
Combustion is a chemical process in which a substance reacts with oxygen to produce heat and usually light.

Q2. What is ignition temperature?
Ignition temperature is the lowest temperature at which a substance catches fire.

Q3. What is the fire triangle?
The fire triangle represents the three essentials for combustion: fuel, oxygen and heat.

Q4. What is calorific value?
Calorific value is the amount of heat produced by burning 1 kg of a fuel, measured in kilojoule per kilogram (kJ/kg).

Q5. Why is this chapter important for exams?
It explains combustion, fuel efficiency and fire safety concepts, which are important for school exams and foundation preparation.

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