Class 8 Combustion and Flame - Questions and Answers

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Combustion and Flame - Questions and Answers

Class 8 Combustion and Flame

Question & Answer

I. Objective Type Questions

A. Choose the correct answer

1. Fuels like kerosene are required to burn a piece of wood. Why cannot wood start burning with a matchstick in normal conditions?
Answer: (c) Matchstick cannot heat the wood to its ignition temperature
2. The table shows ignition temperatures. Which substance will catch fire easily when heated at 50°C?
Substance Ignition Temperature (°C)
A 80
B 39
C 110
D 25
Answer: (b) Substances with ignition temperature below 50°C (B and D)
3. The zone of the candle flame which is moderately hot is called:
Answer: (b) luminous zone
4. Which fuels release oxides of sulphur that mix in rainwater to form acid rain?
Answer: (c) both coal and diesel
5. Which of the following causes acid rain?
Answer: (d) oxides of sulphur and nitrogen
6. In the activity to show heating water in a paper cup, the cup with water does not burn because:
Answer: (c) the cup with water doesn't reach its ignition temperature
7. Which of the following is a combustion reaction?
Answer: (a) Magnesium + Oxygen → Magnesium oxide + Heat + Light
8. Which option correctly matches the given columns?
Answer:
  • Combustible Substance - Paper
  • Carbon dioxide - Fire Fighting
  • Hydrocarbons - Fuel
  • Hottest part of the candle flame - Complete Combustion
  • Calorific value - Kilojoule per Kg

B. Fill in the blanks

1. Gaseous fuels have ______ calorific value than solid fuels.
Answer: higher
2. The outermost nonluminous (blue) zone of the candle flame is ______.
Answer: hottest
3. Respiration is an example of ______ combustion.
Answer: slow
4. ______ is more harmful than carbon dioxide.
Answer: Carbon monoxide
5. Oxides of sulphur cause ______.
Answer: lung congestion

C. State True or False. Also correct the false statements

1. Burning of LPG is slow combustion.
Answer: False
Correction: Burning of LPG is rapid combustion.
2. The substance having higher ignition temperature catches fire easily.
Answer: False
Correction: A substance with lower ignition temperature catches fire easily.
3. When coal is burnt in the presence of insufficient air, it produces only carbon monoxide.
Answer: False
Correction: It mainly produces carbon monoxide along with soot/other products.
4. Greenhouse effect causes melting of polar ice.
Answer: True
5. The rain water containing dissolved oxides of nitrogen and sulphur is called acid rain.
Answer: True

D. Tick (✓) the odd-one out giving reason

1. LPG, Petrol, Water, Kerosene, Paper
Answer: Water — noncombustible; others are fuels
2. Petrol, Biogas, Kerosene, Diesel, Alcohol
Answer: Biogas — gaseous fuel; others are liquids
3. Oxygen, Nitrogen, Carbon dioxide, Carbon monoxide
Answer: Oxygen — supporter of combustion; others are products of combustion
4. Acid rain, Oxides of nitrogen, Oxides of sulphur, Carbon dioxide
Answer: Carbon dioxide — greenhouse gas; the others are acid rain-related
5. Carbon monoxide, Haemoglobin, Incomplete combustion, Nitrogen
Answer: Nitrogen — unrelated; others relate to CO poisoning

E. Assertion–Reason

1. A: LPG, petrol, kerosene are inflammable substances.
R: Substances having low ignition temperature catch fire easily.
Answer: (a) Both true and Reason explains Assertion
2. A: A nonluminous flame is produced when a fuel undergoes complete combustion.
R: A candle flame is uniformly nonluminous.
Answer: (c) Assertion true but Reason false
3. A: Global warming is the gradual increase in the average temperature of earth's atmosphere.
R: Burning of fossil fuel mitigates the effect of global warming.
Answer: (c) Assertion true but Reason false
4. A: Acid rain reacts with limestone and marble and corrodes the structure made with them.
R: The oxides of sulphur and nitrogen along with other pollutants mix with rainwater and make it acidic.
Answer: (a) Both true and Reason explains Assertion
5. A: Despite high calorific value wood is not recommended as a fuel.
R: Burning wood creates a lot of smoke and promotes deforestation.
Answer: (d) Assertion false but Reason true

II. Short Answer Type Questions

1. State necessary conditions for combustion.
Answer: Presence of a combustible substance (fuel), sufficient oxygen supply, and heating to its ignition temperature.
2. What is meant by Luminous and Nonluminous flame?
  • Luminous flame: Yellow flame, incomplete combustion
  • Nonluminous flame: Blue flame, complete combustion, gives maximum heat
3. What are the characteristics of an ideal fuel?
Answer: High calorific value; burns cleanly without poisonous gases or residue.
4. What is acid rain? How does it affect structures?
Answer: Rainwater with dissolved oxides of sulphur and nitrogen; corrodes marble, limestone, and metal structures.
5. Complete the flow chart:
  • X = Solid fuels (e.g., Bagasse)
  • Y = Liquid fuels (e.g., Alcohol)
  • Z = Gaseous fuels (e.g., LPG, Biogas, Natural gas)

III. Long Answer Type Questions

1. What is a combustible substance? Give examples.
Answer: A substance that burns in oxygen/air producing heat and light. Examples: wood, coal, petrol, kerosene, LPG.
2. Why does a spark in a tin can containing petrol cause fire? What would happen if there was no oxygen inside? Why doesn't the same happen with diesel?
Answer:
  • Petrol vapour + air ignites easily with a spark
  • If no oxygen inside, it won't burn
  • Diesel is less volatile; spark usually cannot ignite it
3. How are fuels classified? Give two examples of each.
Answer:
  • Solid: coal, wood
  • Liquid: petrol, kerosene
  • Gaseous: LPG, natural gas
4. What pollutants are produced by burning fossil fuels? How do they affect us?
Answer:
  • CO: poisonous, causes suffocation
  • CO₂: causes global warming
  • SOx: causes acid rain, lung irritation
  • NOx: causes smog, acid rain
  • Smoke/soot: causes pollution, health issues
5. Identify the zones of a candle flame:
  • (a) Hottest zone → Outermost nonluminous blue zone
  • (b) Luminous zone → Middle yellow zone
  • (c) No combustion → Inner dark zone
  • (d) Zone with CO burning → Lower blue zone near base

IV. Higher Order Thinking Skills

1. Which fuel will burn without producing water vapour? Why?
Answer: Carbon (coke/charcoal), as it has no hydrogen to form water vapour.
2. Why does a flame always point upwards?
Answer: Because hot gases are lighter and rise by convection.
3. What would happen if a substance has ignition temperature lower than room temperature?
Answer: The substance may catch fire spontaneously (very hazardous).

V. Concept Based Questions

A. Matchstick Passage
1. Matchstick burns due to → (b) friction
2. Combustible substance → (d) powdered sulphur
3. Cannot catch fire until → (c) ignition temperature is reached
4. Lowest temperature for catching fire → (c) ignition temperature
5. Example of rapid combustion → (c) methane + oxygen reaction
B. Fire Extinguisher Passage
1. Water poured on fire: true are (d) (i), (ii), (iii)
2. Not used for electrical fire because → (a) water conducts electricity
3. CO₂ is excellent extinguisher because → (c) (i) and (iii)
4. Fire triangle: X = (d) heat
C. Oil Pan Fire
Answer: (c) Baking soda releases CO₂ which blankets the burning oil.

Puzzles and Quiz

ACROSS

1. Rise in temperature on the earth (13) → GLOBALWARMING
2. Noncombustible substance with high melting point (4) → SAND
3. A gaseous substance with highest calorific value (8) → HYDROGEN
4. Slow, spontaneous combustion with no light, constant temperature (9) → OXIDATION

DOWN

5. Liquid fuel used as domestic fuel (8) → KEROSENE
1. Supporter of combustion (6) → OXYGEN
2. Gaseous fuel from petroleum (3) → LPG
3. Metallic element burning with dazzling white light (9) → MAGNESIUM
4. Depletion of nonrenewable energy (12) → ENERGYCRISIS

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