Class 8 Combustion and Flame
NCERT Exemplar Problems - Solutions
Multiple Choice Questions
1. A substance which reacts with oxygen giving heat is called a combustible substance. Which one is combustible?
Answer: (d) wood
2. Which has the highest calorific value?
Answer: (c) LPG
3. Magnesium ribbon on burning in air produces:
Answer: (c) magnesium oxide, heat and light
4. Which of the following is not a combustible substance?
Answer: (b) glass
5. The substance that does not burn with a flame is
Answer: (d) charcoal
6. Why does a candle flame go out under an inverted tumbler?
Answer: (a) oxygen
7. Best way to extinguish fire on a person's clothes:
Answer: (c) cover the person with a woolen blanket.
Reason: A woolen blanket cuts off the supply of oxygen to the fire, quickly smothering it. Throwing water may not be effective (especially if synthetic clothes are burning, as they can stick to the skin). Fire extinguishers are usually not safe for direct use on the body, and polythene sheets can melt and worsen the injury.
8. Highest ignition temperature among these:
Answer: (c) coal
9. Correct statement about inflammable substances:
Answer: (c) low ignition temperature and can catch fire easily.
10. Choose the incorrect statement about forest fires:
Answer: (c) cutting of trees
11. Unit of calorific value:
Answer: (d) kilojoule per kilogram
12. Why is wood used as fuel in villages?
Answer: (b) because of its easy availability and low cost.
13. Cleanest fuel:
Answer: (d) hydrogen gas
14. Incorrect statement about a good fuel:
Answer: (c) leaves behind many undesirable substances.
15. Copper vessel blackened while cooking on chulha because of:
Answer: (c) improper combustion of the fuel.
Very Short Answer Questions
16. Fill in the blanks:
(a) A chemical process in which a substance reacts with oxygen to give off heat is called combustion.
(b) When the clothes of a person catch fire, the person is covered with a blanket to extinguish fire.
(c) The lowest temperature at which a substance catches fire is called its ignition temperature.
(d) The substances which have very low ignition temperature and can easily catch fire with a flame are called inflammable substances.
(e) The substances which vapourise during burning give flame.
(b) When the clothes of a person catch fire, the person is covered with a blanket to extinguish fire.
(c) The lowest temperature at which a substance catches fire is called its ignition temperature.
(d) The substances which have very low ignition temperature and can easily catch fire with a flame are called inflammable substances.
(e) The substances which vapourise during burning give flame.
17. Correct the jumbled words:
(a) Seldie → Diesel is a combustible substance.
(b) Slags → Glass is a non-combustible material.
(c) Chittsmack → Matchstick does not burn by itself.
(d) thea and mafel → Some substances on combustion produce heat and flame.
(e) ficalroic → The amount of heat energy produced on complete combustion of 1 kg of a fuel is called its calorific value.
(b) Slags → Glass is a non-combustible material.
(c) Chittsmack → Matchstick does not burn by itself.
(d) thea and mafel → Some substances on combustion produce heat and flame.
(e) ficalroic → The amount of heat energy produced on complete combustion of 1 kg of a fuel is called its calorific value.
18. Two jars: A (CO2) and B (O2). In which will candle burn longer? Why?
Answer: In jar B with oxygen, because oxygen supports combustion while carbon dioxide does not.
19. Which zone of a flame is best for boiling water quickly?
Answer: The outermost blue (non-luminous) zone, because it is the hottest part of the flame.
20. Why is CNG replacing diesel and petrol in big cities?
Answer: CNG burns more cleanly (less smoke and harmful gases) and causes less air pollution.
Short Answer Questions
21. Separate as combustible and non-combustible:
Combustible: Charcoal, straw, cardboard, paper, candle, wood.
Non-combustible: Chalk, stone, iron rod, copper coin, glass.
Non-combustible: Chalk, stone, iron rod, copper coin, glass.
22. True/False and correct:
(a) Air is necessary for combustion. True.
(b) Magnesium is a non-combustible metal. False. Correction: Magnesium is combustible.
(c) Carbon dioxide is an excellent fire extinguisher. True.
(d) Calorific value of wood is higher than that of coal. False. Correction: Calorific value of coal is higher than that of wood.
(b) Magnesium is a non-combustible metal. False. Correction: Magnesium is combustible.
(c) Carbon dioxide is an excellent fire extinguisher. True.
(d) Calorific value of wood is higher than that of coal. False. Correction: Calorific value of coal is higher than that of wood.
23. Match Column A with Column B:
| (a) Oxides of sulphur and nitrogen | → (iv) acid rain |
| (b) CNG | → (vi) fuel for automobiles |
| (c) Oxygen | → (v) necessary for combustion |
| (d) Inflammable substance | → (iii) very low ignition temperature |
| (e) Carbon dioxide | → (i) fire extinguisher |
| (f) Carbon monoxide | → (ii) incomplete combustion of coal |
24. Match for candle flame:
| (a) hottest part | → (iii) outer zone of complete combustion → (x) blue |
| (b) moderately hot | → (ii) middle zone of partial combustion → (z) yellow |
| (c) least hot | → (i) innermost zone of unburnt wax vapours → (y) black |
25. Iron wire held in a flame—what happens? Does it produce a flame?
Answer: The iron wire becomes red hot and may glow but it does not produce a flame because metals generally do not vaporise to form a flame.
26. Fill in the blanks (use words from the box):
(a) A chemical process in which a substance reacts with oxygen to give off heat is called combustion.
(b) Wood, paper, CNG are combustible substances.
(c) The lowest temperature at which a substance catches fire is called its ignition temperature.
(d) Ignition temperature of petrol is lower than that of wood.
(e) The substances which have very low ignition temperature and can easily catch fire with a flame are called inflammable substances.
(f) The amount of heat energy produced on complete combustion of 1 kg of a fuel is called its calorific value.
(b) Wood, paper, CNG are combustible substances.
(c) The lowest temperature at which a substance catches fire is called its ignition temperature.
(d) Ignition temperature of petrol is lower than that of wood.
(e) The substances which have very low ignition temperature and can easily catch fire with a flame are called inflammable substances.
(f) The amount of heat energy produced on complete combustion of 1 kg of a fuel is called its calorific value.
27. Why keep the door open when using angithi/burning coal in a closed room?
Answer: To allow fresh air (oxygen) to enter and prevent build-up of poisonous carbon monoxide, which can cause suffocation and death.
28. True/False and correct:
(a) A physical process in which a substance reacts with oxygen to give off heat is called combustion. False. Correction: It is a chemical process.
(b) Water is the best extinguisher for fires involving electrical equipment. False. Correction: CO2 or dry powder should be used; water can cause electric shock.
(c) Alcohol, CNG and LPG are inflammable substances. True.
(d) Increased concentration of nitrogen in air is believed to cause global warming. False. Correction: Increased carbon dioxide (and other greenhouse gases) causes global warming.
(e) Greater the calorific value, better is the fuel. True.
(f) Middle zone is the hottest zone of a flame. False. Correction: The outermost blue zone is the hottest.
(g) The substances which vapourise during burning, give flame. True.
(b) Water is the best extinguisher for fires involving electrical equipment. False. Correction: CO2 or dry powder should be used; water can cause electric shock.
(c) Alcohol, CNG and LPG are inflammable substances. True.
(d) Increased concentration of nitrogen in air is believed to cause global warming. False. Correction: Increased carbon dioxide (and other greenhouse gases) causes global warming.
(e) Greater the calorific value, better is the fuel. True.
(f) Middle zone is the hottest zone of a flame. False. Correction: The outermost blue zone is the hottest.
(g) The substances which vapourise during burning, give flame. True.
29. Why does a cracker produce sound on ignition?
Answer: Rapid combustion and sudden expansion of hot gases create a shock wave, which we hear as a loud sound.
30. What is meant by fuel efficiency?
Answer: Fuel efficiency tells how much useful energy a fuel gives; fuels with higher calorific value and cleaner, complete combustion are more efficient.
Long Answer Questions
31. You are provided with three watch glasses containing milk, petrol and mustard oil, respectively. Suppose you bring a burning candle near these materials one by one, which material(s) will catch fire instantly and why?
Answer: Petrol will catch fire instantly because it is highly volatile with low ignition temperature. Mustard oil may catch fire on strong heating; milk will not catch fire.
32. Manu was heating oil to fry potato chips. The cooking oil all of a sudden caught fire; he poured water to extinguish the fire. Do you think this action was suitable. If yes, why? If not, why not? In such a condition what should Manu have done?
Answer: Not suitable. Water makes burning oil spread. Instead, cover the pan with a lid or damp cloth to cut air, switch off the flame, or use baking soda/CO2 extinguisher.
33. What are the three essential requirements to produce fire? How fire extinguisher is useful for controlling the fire?
Answer: The three essential requirements to produce fire are:
- Fuel – a combustible substance,
- Oxygen (air) – to support burning,
- Heat – to raise the substance to its ignition temperature.
A fire extinguisher works by removing one or more of these requirements:
- Carbon dioxide (CO₂): cuts off oxygen and cools the fire,
- Water: lowers the temperature and produces steam that displaces oxygen,
- Foam: forms a barrier over the fuel to stop its contact with air.
34. Give two examples each for a solid, liquid and gaseous fuel along with some important uses.
Answer:
- Solid fuels: Coal (power plants), Wood (cooking/heating).
- Liquid fuels: Petrol (cars/bikes), Kerosene (stoves/lamps).
- Gaseous fuels: LPG (cooking), CNG (vehicles).
35. The calorific values of petrol and CNG are 45000 kJ/kg and 50,000 kJ/kg, respectively. If you have vehicle which can run on petrol as well as CNG, which fuel will you prefer and why?
Answer: Prefer CNG because it has higher calorific value and burns more cleanly, causing less air pollution.
36. Although wood has a very high calorific value, we still discourage its use as a fuel.
Answer:
Wood is discouraged as a fuel because:
- Cutting trees for fuel leads to deforestation.
- It produces a lot of smoke and harmful pollutants.
- It leaves behind a large amount of ash after burning.
Thus, it is not a clean or environmentally friendly fuel.
37. Forest fire produces a lot of air pollution. Write in brief about the reasons of forest fires.
Answer:
Forest fires occur due to:
- Natural causes – such as lightning strikes and very high temperatures in summers.
- Human activities – such as unattended campfires, burning leaves, throwing cigarette butts, and careless handling of fire.
Crossword Puzzle Answers:
Across
1. Non-metal which catches fire if exposed to air (10): PHOSPHORUS
2. The lowest temperature at which a substance catches fire is called its ______ temperature (8): IGNITION
3. The most common fire extinguisher (5): WATER
Down
2. A chemical process in which a substance reacts with oxygen to give off heat (10): COMBUSTION
4. Petrol is used as a ______ in automobiles (4): FUEL
6. It is as hard as stone and black in colour (4): COAL
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