Combustion and Flame – Case-based Questions with Answers
CBSE Class 8 Science — Chapter 6: Combustion and Flame
- Read NCERT theory and label key diagrams (flame zones, fire triangle).
- Understand complete vs incomplete combustion and common examples.
- Learn safety measures and extinguisher types; practise case-based reasoning.
- Practice model answers and time-managed writing for long and case-based questions.
20 Case-Based Questions & Model Answers
Case 1 — Yellow Flame on Stove
Ria notices a yellow flame and black soot around her gas stove while cooking. She also smells a faint odour sometimes and sees occasional black deposits on vessels.
Questions:
- What does the yellow flame and soot indicate?
- List two immediate steps Ria should take to make the flame efficient and safe.
Answer: Yellow flame and soot indicate incomplete combustion due to insufficient air supply or dirty burner. Immediate steps: (1) Clean the burner holes and ensure no blockage so proper air mixes with gas. (2) Adjust the air supply or service the stove to ensure a blue flame; also improve ventilation in the kitchen to prevent CO buildup.
Case 2 — Oily Rags in a Workshop
A carpenter leaves oily rags piled in a corner. After some time the pile heats up and catches fire without any external flame.
Questions:
- What phenomenon explains this incident?
- Suggest two safety practices to avoid such fires.
Answer: This is spontaneous combustion — heat from oxidation of oil builds up and ignites the rags. Safety practices: store oily rags spread out or in metal containers with lids, and keep them damp or disposed promptly; ensure good ventilation and avoid piling them together.
Case 3 — Candle Flame Diagram Question
In a class test, students are asked to draw a labelled diagram of a candle flame and explain why the outer zone is hottest.
Questions:
- Mention the three zones and one characteristic of each.
- Explain concisely why outer zone is hottest.
Answer: Zones: Inner (dark) — unburnt vapour, coolest; Middle (luminous) — glowing carbon particles, yellow; Outer (non-luminous) — complete combustion, hottest. Outer zone is hottest because it has the best access to oxygen allowing complete combustion which releases maximum heat.
Case 4 — Car Exhaust and Catalytic Converter
A mechanic explains that a car without a functioning catalytic converter emits more CO and hydrocarbons.
Questions:
- Why does a faulty catalytic converter cause higher pollution?
- What combustion-related issue in engines does the converter address?
Answer: Catalytic converters promote oxidation of CO and unburnt hydrocarbons to CO₂ and H₂O, and reduce NOx; a faulty unit cannot catalyse these reactions, increasing pollutant emissions. It addresses incomplete combustion products from the engine, converting them into less harmful compounds.
Case 5 — Chimney Fire
A householder notices smoke and flames coming from the chimney during winter after prolonged use of an old fireplace.
Questions:
- What is the most probable cause?
- What preventive maintenance could have prevented this?
Answer: Probable cause: accumulation of flammable soot/tar (creosote) in the chimney which ignited. Preventive maintenance: regular chimney sweeping to remove soot, using dry seasoned wood, and ensuring proper draft and combustion efficiency.
Case 6 — Gas Leak Smell
Neha smells gas in the kitchen. She wonders what immediate actions to take before the gas concentration reaches dangerous levels.
Questions:
- List immediate safety steps.
- Why must she avoid using electrical switches or mobile phones near the leak?
Answer: Steps: turn off gas supply if safe, ventilate the area by opening doors and windows, evacuate the room, avoid using any flame or spark-producing devices, and call gas emergency services. Avoid electrical switches or phones because switching can produce sparks that ignite the gas-air mixture.
Case 7 — Yellow Flame in Bunsen Burner Lab
In a chemistry experiment the teacher demonstrates two flames: blue when air hole is open, yellow when closed.
Questions:
- Explain why flame color changes.
- Which flame is preferred for heating and why?
Answer: Colour change due to air (oxygen) supply: open air hole mixes gas with oxygen causing complete combustion (blue flame); closed hole limits oxygen causing incomplete combustion, producing glowing carbon particles (yellow flame). Blue flame is preferred for heating because it is hotter and indicates complete combustion.
Case 8 — Fire Extinguisher Selection
A small oil pan fire starts in a school kitchen. A teacher reaches for an extinguisher but only has water and CO₂ types available.
Questions:
- Which extinguisher should the teacher use and why?
- Why is the other unsuitable?
Answer: Use CO₂ extinguisher — it displaces oxygen and does not spread oil. Water is unsuitable because it can spread burning oil, causing the fire to enlarge and splash burning fuel.
Case 9 — Carbon Monoxide Incident
In a poorly ventilated room with a portable heater, some family members feel dizzy and nauseous.
Questions:
- Suggest the likely cause.
- How can such incidents be prevented?
Answer: Likely CO poisoning from incomplete combustion in the heater due to poor ventilation. Prevention: ensure adequate ventilation, use properly maintained appliances, install CO detectors, and avoid using combustion devices in enclosed spaces.
Case 10 — Smouldering Coal in Coal Yard
A coal yard reports slow burning spots that persist and do not produce flames but emit smoke for days.
Questions:
- Identify the combustion type.
- Suggest control measures for smoke and fire spread.
Answer: Smouldering (flameless) combustion. Control measures: isolate affected piles, reduce oxygen access by covering with soil or sand, carefully remove and cool hotspots, and monitor to prevent re-ignition. Proper storage and avoiding large piles help prevent smouldering.
Case 11 — Laboratory Match Safety
Students are using matches and a teacher reminds them of safe strike and disposal practices.
Questions:
- What safe practices should students follow?
- Why is correct disposal important?
Answer: Practices: strike matches away from body, light away from others, extinguish completely in water or sand, and keep out of children's reach. Proper disposal prevents accidental fires from hot match heads and reduces risk of spontaneous ignition in dustbins with combustible materials.
Case 12 — Stove Maintenence in Restaurant
A restaurant owner notices yellow flames on multiple burners and increased fuel use.
Questions:
- Explain why fuel usage might increase with yellow flames.
- Give two corrective actions.
Answer: Yellow flames indicate incomplete combustion, which releases less energy per unit fuel leading to higher fuel consumption to achieve cooking temperature. Corrective actions: service and clean burners for proper air mixture and train staff to adjust air supply; ensure quality fuel and regular maintenance to restore efficient combustion.
Case 13 — Forest Firebreak
Forest managers create firebreaks by removing strips of vegetation before the dry season.
Questions:
- Explain how removing fuel helps control wildfires.
- What other measure reduces wildfire risk?
Answer: Removing fuel interrupts the continuity of combustible material so fire cannot spread across gaps (breaks the fuel side of the fire triangle). Other measures: controlled burning under supervision to reduce fuel loads, creating buffer zones, and community awareness to prevent accidental ignitions.
Case 14 — Candle Soot on Ceiling
After prolonged use of many candles in a temple, black soot appears on the ceiling.
Questions:
- Why does soot deposit form?
- How can soot formation be reduced?
Answer: Soot forms due to incomplete combustion producing carbon particles that rise and deposit. Reduction: improve ventilation to supply oxygen, use cleaner candles that burn with a blue flame or LED substitutes, trim wicks to reduce excessive smoking, and avoid overcrowding candles in confined spaces.
Case 15 — Using Sand to Extinguish Fire
A mechanic suggests using dry sand to put out a small petrol spill fire on a workshop floor.
Questions:
- Explain why sand is recommended.
- When is sand unsuitable?
Answer: Sand smothers the fire by cutting off oxygen and absorbs heat; it is non-conductive so safer around electrical equipment. Sand is unsuitable for large fires where professional firefighting is needed and for fires where containment of hazardous chemicals requires specialised agents.
Case 16 — Flash Point Concern in Storage
A school lab stores a volatile solvent and the teacher asks students why it is kept in a cool, ventilated cabinet labeled 'flammable'.
Questions:
- Relate flash point concept to safe storage.
- What lab practice prevents accidental ignition?
Answer: Solvents with low flash points produce flammable vapours at low temperatures that can ignite; cool ventilated storage keeps temperature below flash point and prevents vapour accumulation. Prevention: keep containers tightly sealed, avoid open flames and sparks, and store in designated flammable cabinets with proper labeling.
Case 17 — Agricultural Stubble Burning
Farmers burn crop residues to prepare fields quickly, but air quality in nearby towns worsens.
Questions:
- Explain environmental consequences of stubble burning.
- Suggest two alternatives that reduce pollution.
Answer: Consequences: release of particulate matter, CO, NOx and greenhouse gases, causing respiratory problems and regional haze. Alternatives: mulching and incorporating residue into soil, using residue for bioenergy/composting, and mechanized removal to avoid burning.
Case 18 — Watch Glass Soot Test
A student holds a cool watch glass above a flame and finds a black deposit on it.
Questions:
- What does the black deposit indicate?
- How does this simple test relate to incomplete combustion?
Answer: The black deposit is soot (carbon particles), indicating incomplete combustion. The test shows that when combustion is incomplete, solid carbon particles are produced which can deposit on cooler surfaces, demonstrating the presence of incomplete oxidation.
Case 19 — Heater in Enclosed Garage
Someone runs a kerosene heater in a closed garage overnight and wakes up with headache and tiredness.
Questions:
- Explain the likely health hazard and cause.
- Recommend safer alternatives.
Answer: Likely CO poisoning from incomplete combustion in an enclosed space; symptoms include headache and fatigue. Safer alternatives: use electric heaters, ensure proper ventilation, install CO alarms, or avoid using combustion heaters indoors overnight.
Case 20 — School Fire Drill and Learning
The school organizes a fire drill demonstrating extinguisher types and evacuation. Students are asked why different extinguishers are used for different fires.
Questions:
- Explain with examples why extinguisher choice matters.
- Give two general fire safety rules students should remember.
Answer: Extinguisher choice matters because using the wrong agent can worsen the fire (water on oil spreads flames; water on electrical fires conducts electricity). Example: CO₂ for electrical fires, foam for oil fires, water for wood/paper. Safety rules: keep exits clear and follow evacuation procedure; never try to fight large fires—alert adults/emergency services and use nearest suitable extinguisher for small fires only.