Chapter 6: Combustion and Flame

MULTIPLE CHOICE QUESTIONS

1. A substance which reacts with oxygen giving heat is called a combustible substance. Which, one of the following is a combustible substance?
(a) iron nail     (b) glass    (c) stone piece  (d) wood

Answer: (d) wood

[ Wood reacts with oxygen to produce heat and light (combustion), while iron nail, glass and stone piece do not burn easily.]

2.Which one of the following has the highest calorific value?
(a) kerosene   (b) biogas     (c) LPG       (d) petrol

Answer: (c) LPG

[ LPG has a calorific value of about 55,000 kJ/kg, which is higher than kerosene, petrol, and biogas.]

3.Magnesium ribbon on burning in air produces
(a) magnesium oxide, water and light
(b) magnesium oxide and heat
(c) magnesium oxide, heat and light
(d) magnesium oxide, water and heat

Answer: (c) magnesium oxide, heat and light

When magnesium burns in air, it combines with oxygen to form magnesium oxide, releasing heat and light.

4. Which of the following is not a combustible substance?
(a) camphor     (b) glass        (c) straw      (d) alcohol

Answer: (b) glass

Glass does not burn or react with oxygen to produce heat, unlike camphor, straw, and alcohol which are combustible.

5. The substance that does not burn with flame is
(a) LPG      (b) camphor    (c) dry grass    (d) charcoal

Answer: (d) charcoal

[ Charcoal burns only with a red glow (without a flame), while LPG, camphor and dry grass burn with a flame.]

6.On placing an inverted tumbler over a burning candle, the flame extinguishes after some time. This is because of non-availability of
(a) oxygen     (b) water vapours     (c) carbon dioxide   (d) wax

Answer:  (a) oxygen

[ The tumbler traps limited air; once the oxygen inside is used up, the candle flame extinguishes.]

7.If a person’s clothes catches fire, the best way to extinguish the fire is to:
(a) throw water on the clothes.
(b) use fire extinguisher.
(c) cover the person with a woolen blanket.
(d) cover the person with a polythene sheet.

Answer: (c) cover the person with a woolen blanket

[ A woolen blanket cuts off oxygen supply, extinguishing the fire; water or polythene may cause harm.]

8. The substance expected to have the highest ignition temperature out of the following is
(a) kerosene     (b) petrol      (c)  coal       (d) alcohol

Answer:  (c) coal

[ Coal has the highest ignition temperature (around 400–500°C) compared to kerosene, petrol and alcohol, which catch fire more easily.]

9. Choose the correct statement about inflammable substances from the following.They have:
(a) low ignition temperature and cannot catch fire easily.
(b) high ignition temperature and can catch fire easily.
(c) low ignition temperature and can catch fire easily.
(d) high ignition temperature and cannot catch fire easily.

Answer: (c) low ignition temperature and can catch fire easily.

[ Inflammable substances like petrol and alcohol have very low ignition temperatures, so they catch fire easily.]

10. Choose the incorrect statement from the following .Forest fires are usually due to:
(a) carelessness of humans     (b) heat of sun   (c) cutting of trees    (d) lightning strike

Answer: (c) cutting of trees

[ Cutting trees removes fuel but does not cause fires; humans, sun heat or lightning can start forest fires.]

11.The calorific value of a fuel is expressed in a unit called
(a) kilojoule per litre   (b) kilogram per mililitre  (c) kilojoule per gram      (d) kilojoule per kilogram

Answer: (d) kilojoule per kilogram

[ Calorific value is the heat produced by burning 1 kg of fuel, so its unit is kJ/kg.]

12. In villages, people use wood as fuel because:
(a) it is considered to be an ideal fuel.
(b) of its easy availability and low cost.
(c) it is environment friendly.
(d) it catches fire easily.

Answer: (b) of its easy availability and low cost.

[ Wood is widely available in villages and inexpensive, making it a common fuel despite not being ideal. ]

13. Which among the following is considered as the cleanest fuel?
(a) cow dung cake           (b) petrol     (c) kerosene      (d) hydrogen gas

Answer: (d) hydrogen gas

[ Hydrogen burns with water as the only product, causing no pollution, making it the cleanest fuel.]

14.Choose the incorrect statement from the following. A good fuel is one which:
(a) is readily available.
(b) produces a large amount of heat.
(c) leaves behind many undesirable substances.
(d) burns easily in air at a moderate rate.

Answer: (c) leaves behind many undesirable substances.

[ A good fuel should leave minimal or no undesirable residues; leaving many is incorrect for a good fuel.]

15.Shyam was cooking potato curry on a chulha. To his surprise he observed that the copper vessel was getting blackened from outside. It may be due to:
(a) proper combustion of fuel.
(b) improper cooking of potato curry.
(c) improper combustion of the fuel.
(d) burning of copper vessel.

Answer: (c) improper combustion of the fuel.

[ Incomplete burning of fuel produces soot, which blackens the vessel’s outer surface.]

VERY SHORT ANSWER QUESTIONS

16. Fill in the blanks in the following sentences.
(a) A ____________ process in which a substance reacts with __________ to give off heat is called combustion.
(b) When the clothes of a person catch __________, the person is covered with a __________ to extinguish fire.
(c) The __________ temperature at which a substance catches fire is called its __________ temperature.
(d) The substances which have very ________________ ignition temperature and can easily catch fire with a flame are called __________ substances.
(e) The substances which vapourise during __________, give flame.

Answer:  (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.

17. Some words (underlined) in the following sentences are jumbled up. Write them in their correct form.
(a) Seldie is a combustible substance.
(b) Slags is a non-combustible material.
(c) Chittsmack does not burn by itself.
(d) Some substances on combustion produce thea and mafel.
(e) The amount of heat energy produced on complete combustion of 1 kg of a fuel is called its ficalroic value.

Answer: (a) Diesel is a combustible substance.

(b) Glass is a non-combustible material.

(c) Matchstick does not burn by itself.

(d) Some substances on combustion produce heat and flame.

(e) The amount of heat energy produced on complete combustion of 1 kg of a fuel is called its calorific value.

18. Two glass jars A and B are filled with carbon dioxide and oxygen gases, respectively. In each jar a lighted candle is placed simultaneously. In which jar will the candle remain lighted for a longer time and why?

Answer: The candle in jar B (oxygen) will remain lighted longer because oxygen supports combustion, while carbon dioxide in jar A does not.

19. Anu wants to boil water quickly in a test tube. On observing the different zones of the flame, she is not able to decide which zone of the flame will be best for boiling water quickly. Help her in this activity.

Answer: Anu should use the outermost zone (non-luminous zone) of the flame. It is the hottest part due to complete combustion, so it will boil water quickly.

20. Why is the use of diesel and petrol as fuels in automobiles being replaced by Compressed Natural Gas (CNG) in big cities?

Answer: Diesel and petrol cause more air pollution. CNG burns completely, produces fewer harmful gases, is less polluting and helps reduce smog in big cities.

SHORT ANSWER QUESTIONS

21. Boojho wants to separate the following materials as combustible and non-combustible. Can you help him?
Charcoal, chalk, stone, iron rod, copper coin, straw, cardboard, glass, paper, candle, wood.

Answer: Combustible (catch fire easily): Charcoal, straw, cardboard, paper, candle and wood

Non-combustible (do not burn): Chalk, stone, iron rod, copper coin and glass

22. Indicate whether the following statements are True or False. Also write the false statements in their correct form.
(a) Air is necessary for combustion.
(b) Magnesium is a non-combustible metal.
(c) Carbon dioxide is an excellent fire extinguisher.
(d) Calorific value of wood is higher than that of coal.

Answer: (a) True – Air (oxygen) is necessary for combustion.

(b) False – Magnesium is a combustible metal (it burns in air with a bright flame).

(c) True – Carbon dioxide is an excellent fire extinguisher (it cuts off oxygen supply).

(d) False – Calorific value of coal is higher than that of wood (coal: 25,000–33,000 kJ/kg; wood: 17,000–22,000 kJ/kg).

23. Match the items of Column A with the items of Column B.
      Column A                                Column B
(a) Oxides of sulphur and        (i) fire extinguisher nitrogen
(b) CNG                                (ii) incomplete combustion of coal
(c) Oxygen                            (iii) very low ignition temperature
(d) inflammable substance      (iv) acid rain
(e) carbon dioxide                   (v) necessary for combustion
(f) carbon monoxide                (vi) fuel for automobiles.

Answer: (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 the following for the flame of a candle.

Column A	Column B (zone)	Column C (Colour)
(a) hottest part	(i) innermost zone  of unburnt wax vapours	(x) blue
(b) moderately hot	(ii) middle zone of  partial combustion	(y) black
(c) least hot	(iii) outer zone of  complete combustion	(z) yellow

Answer: The correct matching is :

Column A	Column B (zone)	Column C (colour)
(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. If you hold a piece of iron wire with a pair of tongs inside a candle flame or a Bunsen burner flame, what will you observe? Will it produce a flame?

Answer: Iron wire will become red hot and glow, but it will not produce a flame. This is because iron does not vaporize during burning; only substances that vaporize produce a flame. Iron undergoes slow surface combustion, releasing heat and light without a flame.

26. Fill in the blanks using the words given in the box.
  ignition, petrol, combustion, calorific value, combustible, inflammable
(a) A chemical process in which a substance reacts with oxygen to give off heat is called __________.
(b) Wood, paper, CNG are __________ substances.
(c) The lowest temperature at which a substance catches fire is called its __________ temperature.
(d) Ignition temperature of __________ is lower than that of wood.
(e) The substances which have very low __________ temperature and can easily catch fire with a flame are called __________ substances.
(f) The amount of heat energy produced on complete combustion of 1kg of a fuel is called its __________.

Answer: (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 1kg of a fuel is called its calorific value.

27. People usually keep Angethi/burning coal in their closed rooms during winter season. Why is it advised to keep the door open?

Answer: It is advised to keep the door open because burning coal produces carbon monoxide (a poisonous gas) due to incomplete combustion in closed rooms. This gas can cause suffocation, unconsciousness, or even death. Open doors allow fresh air (oxygen) to enter and harmful gases to escape.

28. Write True/False against the following statements and also correct the false statement.
(a) A physical process in which a substance reacts with oxygen to give off heat is called combustion.
(b) Water is the best extinguisher for fires involving electrical equipment.
(c) Alcohol, CNG and LPG are inflammable substances.
(d) Increased concentration of nitrogen in air is believed to cause global warming.
(e) Greater the calorific value, better is the fuel.
(f) Middle zone is the hottest zone of a flame.
(g) The substances which vapourise during burning, give flame .

Answer: (a) False – A chemical process in which a substance reacts with oxygen to give off heat is called combustion.

(b) False – Water is not the best extinguisher for fires involving electrical equipment (it can cause electric shock; carbon dioxide extinguisher is used).

(c) True – Alcohol, CNG and LPG have low ignition temperature and catch fire easily.

(d) False – Increased concentration of carbon dioxide in air is believed to cause global warming.

(e) True – Greater calorific value means more heat per kg, so better the fuel.

(f) False – Outer zone (non-luminous zone) is the hottest zone of a flame.

(g) True – Substances that vapourise during burning produce flame (e.g., kerosene, wax).

29. Cracker on ignition produces sound. Why?

Answer: Crackers produce sound on ignition because they contain gunpowder and other chemicals that undergo rapid combustion, producing large amounts of gases suddenly. The sudden expansion of these gases creates a shock wave, causing a loud sound.
30. What do you understand by fuel efficiency?

Answer: Fuel efficiency refers to the amount of heat energy produced by complete combustion of a fuel. It is measured by calorific value (kJ/kg). A fuel with higher calorific value is more efficient because it produces more heat per unit mass.

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 has a very low ignition temperature and is highly inflammable. Milk (contains water) and mustard oil (higher ignition temperature) do not catch fire as easily as petrol.

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: No, pouring water was not suitable. Water is heavier than oil and sinks below it, so it does not cool the oil effectively. Instead, water turns into steam rapidly, splashing burning oil and spreading the fire.

Manu should have turned off the stove and covered the pan with a metal lid or a damp cloth to cut off the oxygen supply.

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, air (oxygen), and heat (ignition temperature). A fire extinguisher works by removing one of these. It cuts off the oxygen supply (using CO₂) or cools down the fuel below its ignition temperature, thereby controlling the fire.

34. Give two examples each for a solid, liquid and gaseous fuel along with some important uses.

Answer: The two example of solid fuels:

(a) Wood : used for cooking and heating in villages.

(b) Coal : used in thermal power plants to generate electricity.

The two example of liquid fuels:

(a) Petrol: used as fuel in cars and bikes.

(b) Kerosene: used for cooking and in lamps.

The two example of gaseous fuels:

(a) LPG (Liquefied Petroleum Gas) : used for home cooking.

(b) CNG (Compressed Natural Gas) : used as fuel in automobiles.

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: We should prefer CNG because its calorific value (50,000 kJ/kg) is higher than that of petrol (45,000 kJ/kg), meaning it gives more energy per kilogram. Also, CNG burns completely, causes less air pollution, and is cheaper than petrol. So, it is a better and cleaner fuel for vehicles.

36. Although wood has a very high calorific value, we still discourage its use as a fuel. Explain.

Answer: Wood has a high calorific value, yet we discourage its use because:

(a) It causes air pollution by releasing smoke and harmful gases.

(b) It leads to deforestation, harming the environment.

(c) Its burning is incomplete, producing toxic carbon monoxide.

(d) It leaves behind ash, creating disposal problems.

37. Forest fire produces a lot of air pollution. Write in brief about the reasons of forest fires.

Answer: Forest fires are usually caused by:

(i) Carelessness of humans (e.g., thrown cigarettes or campfires left burning).

(ii) Heat of the sun (dry grass and leaves catch fire spontaneously on hot days).

(iii) Lightning strike during storms.

38. Complete the crossword Fig. 6.1 with the help of the clues :

   

Across

1. Non-metal which catches fire if exposed to air (10)

3. The lowest temperature at which a substance catches fire is called its __________ temperature. (8)

5. The most common fire extinguisher. (5)

Down

2. A chemical process in which a substance reacts with oxygen to give off heat. (10)

4. Petrol is used as a __________ in automobiles. (4)

6. It is as hard as stone and black in colour. (4)

Answer:  Across

1. Non-metal which catches fire if exposed to air (10) → Phosphorus .

3. The lowest temperature at which a substance catches fire is called its ignition temperature. (8) → Ignition

5. 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 fuel in automobiles. (4) → Fuel .

6. It is as hard as stone and black in colour. (4) → Coal .