Combustion and Flame – MCQs with Answers and Explanations
CBSE Class 8 Science — Chapter 6: Combustion and Flame
50 Multiple Choice Questions (MCQs) — Topic-wise • NCERT-aligned • Board-exam standard
Class: 8
Subject: Science
Chapter: Combustion & Flame
CBSE Board Examinations (Study order):
- Study NCERT text and diagrams carefully.
- Memorize key definitions and flame zones.
- Understand types of combustion and their consequences.
- Learn fire safety methods and extinguisher uses.
- Practice these MCQs topic-wise and revise model explanations.
Topic 1 — Basics of Combustion (Q1–Q10)
- Q1. Combustion is best described as:
Answer: BExplanation: Combustion involves chemical oxidation with heat and often light (e.g., burning wood).
- Q2. The substance that burns is called:
Answer: CExplanation: Fuel is the material that undergoes combustion (e.g., petrol, wood).
- Q3. Which of the following is NOT required for combustion?
Answer: DExplanation: Fuel, oxygen and sufficient heat are essential. A catalyst is not necessary to start combustion.
- Q4. Ignition temperature is:
Answer: BExplanation: Ignition temperature is the lowest temperature at which a substance catches fire.
- Q5. Which of the following equations represents complete combustion of methane?
Answer: BExplanation: Complete combustion needs sufficient oxygen producing CO₂ and H₂O.
- Q6. Which product indicates incomplete combustion?
Answer: CExplanation: Carbon monoxide (CO) forms when oxygen is limited; CO is poisonous.
- Q7. Flame is visible because of:
Answer: BExplanation: Flame emits light due to excited atoms/particles and glowing carbon particles in case of incomplete combustion.
- Q8. Smouldering combustion is characterised by:
Answer: BExplanation: Smouldering is slow and often flameless, e.g., glowing charcoal or underground coal fires.
- Q9. Spontaneous combustion may occur due to:
Answer: BExplanation: Self-heating from oxidation (e.g., oily rags) can raise temperature to ignition point.
- Q10. Which of these is a correct safety precaution around open flames?
Answer: CExplanation: Proper ventilation and removing combustibles reduce fire risk; water is not suitable for all fires.
Topic 2 — Types of Combustion (Q11–Q20)
- Q11. Rapid combustion is typically seen in:
Answer: BExplanation: Rapid combustion like paper burning releases heat and light quickly.
- Q12. Explosive combustion differs from rapid combustion because:
Answer: CExplanation: Explosive combustion is extremely fast, often producing blasts and high pressure.
- Q13. Flaming combustion and smouldering combustion mainly differ in:
Answer: AExplanation: Smouldering lacks visible flame; flaming combustion shows flame.
- Q14. Which of the following is an example of spontaneous combustion?
Answer: BExplanation: Spontaneous combustion occurs without external ignition source due to internal heat build-up.
- Q15. Flameless combustion typically produces:
Answer: CExplanation: Smouldering or flameless combustion yields glowing and smoke without flame.
- Q16. Which is LEAST likely to cause explosive combustion?
Answer: CExplanation: Controlled open burns are less likely to cause explosive conditions compared to confined dust/gas mixtures.
- Q17. Which combustion type is commonly associated with underground coal seams?
Answer: CExplanation: Underground coal fires are smouldering, slow and flameless.
- Q18. A material that ignites at room temperature without external flame most likely has:
Answer: BExplanation: Low flash point indicates vapour can ignite at low temperatures; self-heating can also cause spontaneous ignition.
- Q19. Which practice reduces risk of spontaneous combustion in stored materials?
Answer: BExplanation: Wetting or sealing in metal containers prevents oxygen access and dissipates heat, reducing ignition risk.
- Q20. Which of these is true about flameless combustion?
Answer: CExplanation: Flameless smouldering in coal seams can persist for long periods with little visible flame.
Topic 3 — Flame Structure (Q21–Q30)
- Q21. The innermost zone of a candle flame contains:
Answer: BExplanation: Inner (dark) zone has unburnt vapour and is relatively cool.
- Q22. The luminous yellow part of the flame is due to:
Answer: BExplanation: Tiny glowing carbon particles formed during partial combustion emit yellow light.
- Q23. Which zone of the flame is the hottest?
Answer: CExplanation: Outer non-luminous zone has sufficient oxygen and shows complete combustion making it hottest.
- Q24. The wick of a candle primarily functions to:
Answer: BExplanation: Capillary action in wick draws molten wax to the flame where it vaporises and burns.
- Q25. If you cover a candle with a jar, the flame goes out because:
Answer: BExplanation: Enclosed jar reduces oxygen supply and extinguishes the flame.
- Q26. The colour of the outer flame is often:
Answer: BExplanation: Blue indicates complete combustion and higher temperature in the outer zone.
- Q27. A student observes soot depositing on a cold spoon held above a flame. This shows:
Answer: BExplanation: Soot deposits indicate carbon particles from incomplete combustion.
- Q28. The temperature gradient in a flame exists because:
Answer: BExplanation: Availability of oxygen increases towards outer zone enabling higher temperatures there.
- Q29. The luminous zone appears bright because:
Answer: BExplanation: Carbon particles heat up and glow, producing yellow light in the luminous zone.
- Q30. For neat diagrams in exams, students should label:
Answer: BExplanation: Labeling inner, middle and outer zones with brief descriptions shows full understanding.
Topic 4 — Complete vs Incomplete Combustion (Q31–Q40)
- Q31. Complete combustion requires:
Answer: CExplanation: Ample oxygen ensures fuel oxidises fully to CO₂ and H₂O.
- Q32. Which is an example of incomplete combustion?
Answer: BExplanation: Candle producing soot indicates incomplete combustion due to limited oxygen.
- Q33. Which gas is a silent killer produced during incomplete combustion?
Answer: CExplanation: CO is colourless, odourless and binds with haemoglobin causing poisoning.
- Q34. Why does incomplete combustion produce less heat?
Answer: BExplanation: Incomplete oxidation converts less chemical energy to heat compared to complete oxidation.
- Q35. The presence of soot indicates:
Answer: CExplanation: Soot is formed from unburnt carbon particles in incomplete combustion.
- Q36. Which of these reduces carbon monoxide emissions in vehicles?
Answer: BExplanation: Catalytic converters oxidise CO to CO₂, reducing toxic emissions.
- Q37. A yellow flame in a gas stove indicates:
Answer: BExplanation: Yellow flame and soot are signs of incomplete combustion due to limited air supply.
- Q38. Which action will help make combustion more complete?
Answer: CExplanation: Better mixing ensures fuel molecules meet oxygen for efficient combustion.
- Q39. Incomplete combustion in household heaters can be reduced by:
Answer: BExplanation: Ventilation provides oxygen; maintenance ensures burners function properly reducing incomplete combustion.
- Q40. Which of the following is NOT a product of incomplete combustion?
Answer: CExplanation: CO₂ is the main product of complete combustion, though it may also be present in incomplete combustion mixes.
Topic 5 — Fire Safety, Extinguishers & Applications (Q41–Q50)
- Q41. The fire triangle consists of:
Answer: AExplanation: Fuel, oxygen and heat are necessary for sustaining fire; removing one extinguishes fire.
- Q42. Water extinguisher should NOT be used on:
Answer: CExplanation: Water conducts electricity and spreads oil fires; unsuitable for electrical/oil fires.
- Q43. Foam extinguishers are especially useful for:
Answer: CExplanation: Foam blankets oil spills cutting off oxygen and preventing re-ignition.
- Q44. CO₂ extinguishers are preferred for electrical fires because:
Answer: BExplanation: CO₂ suffocates flames and does not conduct electricity, making it safe for electrical fires.
- Q45. To reduce air pollution from combustion, one should:
Answer: BExplanation: Cleaner fuels and efficient combustion reduce CO, soot and unburnt hydrocarbons.
- Q46. In internal combustion engines, improving combustion efficiency leads to:
Answer: CExplanation: Efficient combustion extracts more energy per unit fuel, improving mileage and reducing pollutants.
- Q47. Which is an immediate action when smelling gas leak?
Answer: BExplanation: Avoid sparks, ventilate and turn off gas; do not create ignition sources.
- Q48. Using damp wood as fuel will generally:
Answer: CExplanation: Moisture absorbs heat to evaporate, reducing temperature and causing incomplete combustion and smoke.
- Q49. Which of the following reduces particulate emissions from combustion?
Answer: AExplanation: Electrostatic precipitators capture particulate matter from flue gases, reducing emissions.
- Q50. Which safety measure is NOT recommended for preventing fires?
Answer: BExplanation: Storing flammables near heat increases fire risk; other options are recommended safety measures.
These MCQs and explanations are strictly aligned with NCERT content for CBSE Class 8 Chapter 6 and are crafted to aid conceptual clarity and exam preparation.