DAV Class 8 Combustion - Q & A

Combustion

🔥 Important Questions & Answers (DAV)

Questions & Answers

1. What is a fuel? Give examples.
   Materials which produce heat energy on burning in air are called fuels.
   Examples – coal, coke, kerosene, LPG, petrol, and wood.
2. Why are fuels important?
   Because they produce heat energy on burning, which is used for various purposes like cooking, heating, and running machines.
3. Why do some materials produce a flame on burning while others do not? Give examples.
   Some fuels burn with a flame (e.g., candle, kerosene), while others burn without a flame (e.g., coal, coke).
   This depends on the way they vaporize and burn.
4. Define combustion.
   The chemical process in which a substance burns in air (or oxygen) with the release of heat and light energy is called combustion.
5. What is a combustible substance? Give examples.
   A substance which catches fire easily on heating in the presence of air/oxygen and produces heat and light energy is called a combustible substance or fuel.
   Examples: paper, wood, kerosene.
6. Can fuels exist in different states?
   Yes. A fuel may be solid (coal, wood), liquid (kerosene, petrol), or gaseous (LPG, CNG).
7. What is the fuel for our body?
   Food is the fuel for our body. Energy is produced during the ‘combustion’ (oxidation) of food inside our cells.
8. Classify the following into combustible and non-combustible: Matchstick, Paper, Cloth, Rubber, Iron nails, Glass, Straw.
   • Combustible: Matchstick, Paper, Cloth, Rubber, Straw
   • Non-combustible: Iron nails, Glass
9. What are the conditions required for combustion?
   For combustion to take place, three conditions are necessary:
   1. Presence of a combustible substance.
   2. Continuous supply of oxygen (air).
   3. The temperature of the substance should be above its ignition temperature.
10. Why is oxygen called the ‘supporter of combustion’?
    Because oxygen, present in air, helps in burning of fuels. Without oxygen, combustion cannot continue.
11. What is observed when a candle is covered with a glass chimney?
    1. With free air supply → candle burns smoothly.
    2. With less air → flame flickers and produces smoke (sooty flame).
    3. With no air → flame gets extinguished.
12. What is ignition temperature?
    The lowest temperature at which a combustible substance catches fire is called its ignition temperature (or kindling temperature).
13. Why does a paper cup filled with water not catch fire immediately when placed on a flame?
    Because water absorbs the heat supplied, preventing the paper from reaching its ignition temperature. Hence, it does not catch fire.
14. Why does an empty paper cup catch fire immediately?
    Because its temperature rises quickly and reaches its ignition temperature.
15. Why can a piece of paper catch fire with a matchstick, but wood cannot?
    Because paper has a low ignition temperature while wood has a higher ignition temperature, which a single matchstick cannot provide.
16. Explain with examples that different substances have different ignition temperatures.
    1. Paper has a low ignition temperature – burns easily with a match.
    2. Wood has a high ignition temperature – does not burn with a single matchstick.
    3. White phosphorus has a very low ignition temperature (35°C) – it catches fire on its own in summers.
17. Why does a matchstick catch fire only when rubbed on a rough surface?
    Because friction generates heat, which raises the temperature of the chemicals (red phosphorus etc.) at the tip to ignition temperature. This lights the matchstick.
18. Why does blowing over a candle flame extinguish it?
    1. The air current lowers the temperature of burning wax vapours below ignition point.
    2. The carbon dioxide in our breath acts as a fire extinguisher.
19. How did ancient Egyptians make matchsticks?
    They coated pine wood with sulphur. When rubbed, friction raised the temperature of sulphur above its ignition temperature, igniting the stick.
20. What are the causes of forest fires?
    • Human negligence, like not extinguishing cooking fire.
    • Carelessly throwing burning matchsticks or cigarettes on dry grass.
21. Why does a substance burn faster in pure oxygen than in air?
    Because pure oxygen supports combustion more strongly. In pure oxygen, a substance burns about five times faster than in air.
22. Why does the sun produce energy?
    The sun’s mass (98%) is made of hydrogen at very high temperature. Hydrogen atoms combine to form helium (nuclear fusion), releasing enormous heat and light energy.
23. How much energy does the sun produce compared to coal?
    The energy produced by the sun in one hour is about 10⁷ times more than the energy produced by burning 1000 kg of coal.
24. What is slow combustion? Give examples.
    When a combustible material burns at a slow or moderate rate, it is called slow combustion.
    It usually occurs due to insufficient supply of air, so combustion is incomplete.
    Examples: burning of cow dung cakes, wood.
25. What is rapid combustion? Give examples.
    When a combustible material burns at a fast rate, it is called rapid combustion.
    It requires sufficient air supply, and combustion is complete.
    Examples: burning of gaseous fuels like LPG, CNG.
26. What is spontaneous combustion? Give examples.
    When a combustible substance catches fire on its own, even at room temperature, it is called spontaneous combustion.
    Examples: white phosphorus, sodium metal.
27. What are inflammable substances?
    Substances with low ignition temperature that catch fire easily with a flame are called inflammable substances.
    Examples: petrol, alcohol, LPG.
28. What is explosive combustion? Give an example.
    When a mixture of a combustible material and air burns completely in a very short time in a closed space, releasing a huge amount of heat, light, and sound, it is called explosive combustion.
    Example: bursting of crackers (explosion).
29. What are the three conditions necessary for combustion?
    1. Presence of a combustible material.
    2. Continuous supply of air (oxygen).
    3. Temperature of combustible material must be above its ignition temperature.
30. How is this principle used in firefighting?
    A fire can be stopped by:
    • Removing combustible substance,
    • Cutting off air supply, or
    • Lowering temperature below ignition temperature.
31. Give precautions to prevent fire in petrol stations, LPG godowns, etc.
    • Do not bring burning materials near petrol, diesel, or LPG.
    • Avoid careless throwing of burning matchsticks.
    • Take safety precautions in cracker factories, ammunition depots, and homes.
32. How can a fire be extinguished?
    • By cutting air supply (e.g., wrapping in blanket, covering with sand).
    • By lowering temperature below ignition temperature (using water in safe cases).
    • Using carbon dioxide fire extinguisher for oil/gas fires.
33. Why should water not be used to extinguish electrical fires?
    Because water conducts electricity and may cause severe electric shocks. Instead, switch off the main supply and call fire brigade.
34. Why should a person whose clothes have caught fire be wrapped in a blanket?
    Wrapping in a thick blanket cuts off the supply of air, so the fire stops.
35. Why is sand used to extinguish fire caused by petrol or kerosene?
    Sand cuts off the supply of air and stops the burning of petrol/kerosene.
36. How does water extinguish fire?
    Water lowers the temperature of the burning material below its ignition temperature.
37. Why should water not be used to extinguish oil fires?
    Because oil floats on water and the fire may spread instead of stopping.
38. Which type of fire extinguisher is used for oil or gas fires?
    A carbon dioxide (CO₂) fire extinguisher is used.
39. Which type of fire extinguisher is safe for electrical fires?
    A carbon dioxide (CO₂) fire extinguisher is safe.
40. How does a carbon dioxide fire extinguisher work?
    • Contains acid in a glass bottle and sodium bicarbonate outside.
    • When struck, acid mixes with sodium bicarbonate to release CO₂ gas.
    • CO₂ covers the burning material like a blanket, cutting off oxygen supply.
    • It also cools the fuel as CO₂ expands when released.
41. What happens during incomplete combustion?
    1. Unburnt carbon particles (soot) are released – wasting fuel and polluting air.
    2. Carbon monoxide (CO) gas is produced – highly poisonous, can be fatal.
42. Define a flame.
    A flame is the region over which gases burn. Example: burning candle, burning paper.
43. What are the three zones of a candle flame?
    1. Innermost zone (dark): Unburnt wax vapours; coldest part.
    2. Middle zone (luminous, yellow): Partial combustion, contains unburnt carbon particles, moderately hot.
    3. Outermost zone (non-luminous): Complete combustion, blue, hottest part.
44. Which part of a candle flame is the hottest?
    The outermost non-luminous zone – because complete combustion takes place here.
45. Which part of the flame is the coldest?
    The innermost zone, as it contains unburnt wax vapours.
46. Which part of the flame produces soot?
    The luminous (yellow) middle zone – due to unburnt carbon particles.
47. How can we prove that wax vapours are present in the innermost zone?
    By inserting a glass tube in the inner zone – vapours come out and catch fire when a matchstick is brought near them.
48. How can we prove that luminous zone contains unburnt carbon particles?
    By holding a glass slide in the luminous zone – black deposit (soot) forms due to carbon particles.
49. How can we prove that the outermost zone is the hottest?
    By placing a copper wire in different flame zones – it turns red hot in the outermost zone, but not in the luminous zone.
50. How do goldsmiths use the hottest zone of flame?
    They use a metallic blow pipe to direct the non-luminous outer flame on gold. Its temperature (~1300°C) melts gold at specific points to shape ornaments.
51. What is calorific value of a fuel?
    The amount of heat energy produced on completely burning 1 kg of a fuel in pure oxygen.
    Unit: kilojoules per kilogram (kJ/kg).
52. What are the characteristics of a good fuel?
    A good fuel should:
    1. Be cheap, easily available, and transportable.
    2. Be easy to store.
    3. Have high calorific value.
    4. Not produce harmful gases.
    5. Have a suitable ignition temperature (not too low, not too high).
    6. Burn completely and controllably.
53. Why are gaseous fuels considered the best?
    1. Easy transportation in cylinders/pipelines.
    2. Release large energy without ash/residue.
    3. Produce least harmful gases.
    4. Easily ignited and combustion controlled by valve.
54. What are the harmful effects of excessive fuel consumption?
    1. Release of unburnt carbon → breathing problems, smog.
    2. Carbon monoxide formation → poisonous, fatal.
    3. Carbon dioxide release → global warming, melting glaciers, floods.
    4. Sulphur dioxide release → acid rain, damage to buildings, plants, trees.
55. Why is CNG considered a cleaner fuel compared to petrol and diesel?
    Because CNG produces less harmful products, does not release ash, and causes less pollution, making it more eco-friendly.
56. Calorific value of wood is 18,000 kJ/kg. How much wood is required to produce 360,000 kJ of heat energy?
    Given:
    • Calorific value of wood = 18,000 kJ/kg
    • Heat required = 360,000 kJ
    
    Mass of wood required = Heat required / Calorific value
    
    = 360,000 / 18,000 = 20 kg
    
    ✅ Answer: 20 kg of wood is required.
57. Why are gaseous fuels considered better than liquid or solid fuels?
    ✅ Answer:
    • Gaseous fuels burn completely and do not leave ash or smoke.
    • They have high calorific value.
    • They can be easily transported through pipelines.
    • They are easy to ignite and control.
    • They cause less pollution compared to solid or liquid fuels.
58. Why do we say that "burning of fuels" can cause "health hazards"?
    ✅ Answer:
    • Burning of fuels releases harmful gases like carbon monoxide, sulphur dioxide, nitrogen oxides, and unburnt carbon particles.
    • These can cause respiratory problems, headaches, lung diseases, eye irritation, and even cancer on long exposure.
    • Incomplete combustion of fuels also reduces oxygen level and can be fatal.

Notes

Precautions for using CNG in vehicles:

• Regularly check for leakage
• Switch off ignition during refueling
• Ensure proper ventilation
• Avoid smoking near CNG stations

Waste combustors (hospital/industrial):

• Work on the principle of high-temperature incineration
• Destroy harmful pathogens and toxic waste
• Effects: Reduce solid waste volume but produce harmful gases (dioxins, CO₂, particulates) which can cause air pollution

Types of Fire Extinguishers (and common school locations):

• Water extinguisher – for wood, paper fires (kept in classrooms, libraries)
• Foam extinguisher – for petrol, oil fires (kept in labs, parking area)
• CO₂ extinguisher – for electrical fires (kept in computer lab, office, near wiring)
• Dry chemical extinguisher – for all types (general use in corridors, labs)

Soda–Acid Fire Extinguisher

• Type of extinguisher: Chemical fire extinguisher
• Contents:
  • Sodium bicarbonate (NaHCO₃) solution
  • Small bottle containing concentrated sulphuric acid (H₂SO₄)
• Working principle:
  • When the extinguisher is inverted, the glass bottle of sulphuric acid breaks
  • Acid reacts with sodium bicarbonate solution → Carbon dioxide (CO₂) gas is produced
  • CO₂ comes out with pressure and forms a blanket over fire
• Chemical reaction: NaHCO₃ + H₂SO₄ → Na₂SO₄ + H₂O + CO₂ ↑
• Action of CO₂:
  • Being heavier than air, CO₂ covers the fire and cuts off the oxygen supply
  • This smothers the fire and extinguishes it
• Suitable for:
  • Fires caused by wood, paper, cloth (solid combustible materials)
• Not suitable for:
  • Oil (petroleum) fires or electrical fires → since liquid spread and electric shocks may occur