SCIENCE CLASS 8 CHAPTER 6 COMBUSTION AND FLAME
SCIENCE – CLASS 8
CHAPTER 6: COMBUSTION AND FLAME
NCERT Questions and Answers
A. Very Short Answer Questions
Q1. What is combustion?
Answer: Combustion is a chemical process in which a substance reacts with oxygen and releases heat, usually with light or a flame.
Q2. What is a combustible substance?
Answer: A substance that can catch fire and burn under suitable conditions is called a combustible substance.
Examples: Wood, paper and petrol.
Q3. What is a non-combustible substance?
Answer: A substance that does not burn under ordinary conditions is called a non-combustible substance.
Examples: Stone and sand.
Q4. What is ignition temperature?
Answer: The lowest temperature at which a substance catches fire and begins to burn is called its ignition temperature.
Q5. What is a flame?
Answer: A flame is the visible, hot region of burning gases produced during certain types of combustion.
Q6. Name the three essential conditions for fire.
Answer:
Fuel
Oxygen
Sufficient heat to reach the ignition temperature
Q7. What is a fuel?
Answer: A fuel is a substance that releases useful energy when it burns.
Examples: Wood, coal, LPG and natural gas.
Q8. What is calorific value?
Answer: Calorific value is the amount of heat produced by the complete combustion of one kilogram of a fuel. It is commonly expressed in kilojoules per kilogram (kJ/kg).
Q9. Name two inflammable substances.
Answer: Petrol and alcohol are examples of inflammable substances.
Q10. Which gas is necessary for combustion?
Answer: Oxygen is necessary for ordinary combustion.
B. Short Answer Questions
Q11. What are the different types of combustion?
Answer: The main types of combustion discussed in this chapter are:
Rapid combustion: A substance burns rapidly and produces heat and light when an ignition source is provided. Example: LPG burning in a gas stove.
Spontaneous combustion: A substance catches fire without an external flame or spark under suitable conditions. Example: Certain materials can self-heat and ignite.
Explosion: A sudden reaction that releases energy rapidly, often producing heat, light, sound and gases.
Q12. What is the difference between combustible and non-combustible substances?
Answer:
Combustible substances | Non-combustible substances |
|---|---|
Can burn under suitable conditions. | Do not burn under ordinary conditions. |
Release energy during combustion. | Do not ordinarily undergo combustion. |
Examples: Wood and paper. | Examples: Stone and sand. |
Q13. Why does a matchstick catch fire when rubbed against the side of a matchbox?
Answer: Rubbing produces heat due to friction. This heat helps raise the chemicals on the match head to their ignition temperature, causing the matchstick to ignite.
Q14. Why does a piece of wood not catch fire on its own at room temperature?
Answer: Wood requires sufficient heat to reach its ignition temperature. At room temperature, it generally does not receive enough heat to start burning.
Q15. Why is water commonly used to extinguish fires?
Answer: Water cools burning materials and lowers their temperature below the ignition temperature. It is suitable for many ordinary solid-material fires, but it must not be used on electrical equipment that may be live or on burning oil and petrol.
Q16. Why should water not be used to extinguish an oil fire?
Answer: Oil floats on water. Pouring water onto burning oil can spread the burning oil and make the fire more dangerous. A suitable fire extinguisher should be used instead.
Q17. Why is carbon dioxide used in fire extinguishers?
Answer: Carbon dioxide helps extinguish fires by reducing the availability of oxygen around the flames. It is particularly useful for certain electrical and flammable-liquid fires when the appropriate extinguisher is selected.
Q18. What is the difference between complete and incomplete combustion?
Answer:
Complete combustion: Occurs when a fuel burns with sufficient oxygen, producing products such as carbon dioxide and water for many hydrocarbon fuels.
Incomplete combustion: Occurs when oxygen is insufficient, potentially producing carbon monoxide, soot and other harmful products.
Q19. What are the characteristics of an ideal fuel?
Answer: An ideal fuel should:
Have a high calorific value.
Be readily available and affordable.
Be easy and safe to store and transport.
Burn at a controllable rate.
Produce minimal smoke and harmful pollutants.
Leave little or no residue.
Q20. What is global warming?
Answer: Global warming is the long-term increase in Earth's average surface temperature, mainly caused by the rise in greenhouse gases due to human activities.
C. Structure of a Flame
Q21. Name the different zones of a candle flame.
Answer: A candle flame has three main zones:
Outer zone: The hottest zone, where complete combustion generally occurs.
Middle zone: The luminous yellow zone, where incomplete combustion may occur.
Inner zone: The relatively dark zone containing unburnt wax vapours; it is the least hot zone.
Q22. Which zone of a candle flame is the hottest?
Answer: The outermost zone is the hottest because combustion is more complete there due to the availability of oxygen.
Q23. Why does the middle zone of a candle flame appear yellow?
Answer: Tiny soot particles become hot and glow, producing the yellow luminous appearance of the middle zone.
Q24. Why is the innermost zone of a candle flame relatively dark?
Answer: The innermost zone contains unburnt wax vapours and has less complete combustion, so it emits relatively little visible light.
Q25. Why do goldsmiths use the outer zone of a flame for heating metals?
Answer: The outer zone is the hottest region of the flame, making it suitable for heating metals.
D. Long Answer Questions
Q26. Explain the conditions necessary for combustion.
Answer: Three conditions are required for ordinary combustion:
Fuel: A combustible material must be present.
Oxygen: Oxygen or another suitable oxidising agent must be available.
Heat: The fuel must reach its ignition temperature.
Removing any one of these conditions can stop combustion. For example, a fire blanket can help cut off the oxygen supply to a small contained fire, while cooling can reduce the temperature below the ignition point.
Q27. Explain the different methods of extinguishing fire.
Answer: Fire can be extinguished by removing one or more of the conditions necessary for combustion.
Cooling: Water can cool suitable burning materials.
Cutting off oxygen: Fire blankets or suitable foam can help prevent oxygen from reaching the fuel.
Removing fuel: Shutting off a fuel supply, when safe to do so, can stop the fire from continuing.
Using carbon dioxide: Appropriate CO₂ extinguishers can be used for certain electrical and flammable-liquid fires.
The method must be chosen according to the type of fire. Never put yourself at risk trying to extinguish a large or spreading fire; move away and contact emergency services.
Q28. Explain the advantages and disadvantages of fuels.
Answer:
Advantages:
Fuels provide energy for cooking and heating.
They power vehicles and machines.
They support electricity generation.
They are important for industrial processes.
Disadvantages:
Burning fuels can cause air pollution.
Fossil fuels release carbon dioxide.
Incomplete combustion can produce poisonous carbon monoxide.
Some fuels produce smoke, soot and other harmful substances.
Fossil fuel reserves are limited.
Q29. What is the importance of calorific value?
Answer: Calorific value helps us compare the energy released by different fuels. A fuel with a higher calorific value releases more heat per kilogram when completely burned.
For example, fuels with high calorific values can be useful where a large amount of energy is required from a relatively small quantity of fuel.
Q30. What are the harmful effects of burning fuels?
Answer:
Carbon dioxide contributes to global warming.
Carbon monoxide can cause serious poisoning.
Sulphur dioxide and nitrogen oxides contribute to air pollution.
Soot and fine particles can harm the respiratory system.
Smoke and pollutants can damage plants, buildings and ecosystems.
E. Fill in the Blanks
The chemical process of burning is called combustion.
The lowest temperature at which a substance catches fire is its ignition temperature.
Oxygen supports ordinary combustion.
The amount of heat released by complete combustion of one kilogram of fuel is its calorific value.
The hottest zone of a candle flame is the outer zone.
Carbon dioxide is used in certain types of fire extinguishers.
Burning a fuel with insufficient oxygen may cause incomplete combustion.
Petrol is an example of an inflammable substance.
A fuel should ideally have a high calorific value.
The yellow luminous region of a candle flame is its middle zone.
F. True or False
Oxygen is necessary for ordinary combustion. — True
Water is suitable for extinguishing every type of fire. — False
The outer zone of a candle flame is the hottest. — True
Carbon monoxide is a harmless gas. — False
Petrol is an inflammable substance. — True
Incomplete combustion can produce soot. — True
Calorific value is commonly measured in kJ/kg. — True
Removing fuel can help extinguish a fire. — True
The inner zone of a candle flame is the hottest. — False
Burning fossil fuels can contribute to global warming. — True
G. Important NCERT Exercise-Based Questions
Q31. Why is LPG considered a good domestic fuel?
Answer: LPG has a relatively high calorific value, burns efficiently when supplied with sufficient air, produces little smoke under proper conditions and is convenient to store and transport in suitable cylinders.
Q32. Why is it dangerous to burn coal in a closed room?
Answer: Burning coal in a poorly ventilated room can lead to a build-up of carbon monoxide, a poisonous gas, especially if combustion is incomplete. This can cause severe poisoning. Fuel-burning appliances should be used only with appropriate ventilation and safety measures.
Q33. What is the difference between a flame and glowing combustion?
Answer: A flame is the visible region of burning gases. Glowing combustion occurs when a solid material burns or becomes incandescent without producing a distinct flame, as may happen with charcoal.
Q34. Why is fuel efficiency important?
Answer: Efficient fuels provide more useful energy for the amount consumed. This reduces fuel costs, conserves resources and can lower pollution when the fuel is burned cleanly.
Q35. What precautions should we take to prevent fires?
Answer:
Keep matches and lighters away from children.
Keep flammable liquids away from heat sources.
Do not overload electrical sockets.
Turn off gas appliances after use.
Keep suitable fire extinguishers in appropriate locations.
Learn safe evacuation procedures and never re-enter a burning building.
H. Quick Revision Table
Term | Meaning |
|---|---|
Combustion | Chemical reaction that releases heat |
Fuel | Substance used to obtain energy |
Ignition temperature | Lowest temperature at which a substance catches fire |
Calorific value | Heat released by complete combustion of one kilogram of fuel |
Inflammable substance | Substance that catches fire easily |
Rapid combustion | Combustion that occurs rapidly after ignition |
Spontaneous combustion | Ignition without an external flame or spark |
Flame | Visible, hot region of burning gases |
Complete combustion | Burning with sufficient oxygen |
Incomplete combustion | Burning with insufficient oxygen |
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