NCERT Exemplar Class 8 Science Solutions 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

Ans: (d) In the presence of oxygen, wood will catch fire. It is a combustible substance that generates energy, heat, and light when ignited.

2. Which one of the following has the highest calorific value?

(a) kerosene 

(b) biogas

(c) LPG 

(d) petro

Ans: (c) The calorific value of LPG (liquid Petroleum Gas) is 55000 kj/kg. 

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

Ans: (c) Magnesium ribbon has a high reactivity. It produces magnesium oxide, light, and heat as it burns.

4. Which of the following is not a combustible substance?

(a) camphor 

(b) glass 

(c) straw

(d) alcohol

Ans: (b) Glass is a non-flammable material. When noncombustible materials burn, they do not emit light.

5. The substance that does not burn with flame is

(a) LPG 

(b) camphor

(c) dry grass

(d) charcoal 

Ans: (d) Because charcoal is a solid fuel, it does not burn with fuel. The reason for this is that it does not evaporate when heated. It produces no flame and just glows when burned.

6. On placing an inverted tumbler over a burning candle, the flame extinguishes after some time. This is because of unavailability of

(a) oxygen 

(b) water vapours 

(c) carbon dioxide

(d) wax

Ans: (a) After some time of burning, the oxygen or air existing in the tumbler exhaust. The flame goes out because there isn't enough oxygen.

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 a fire extinguisher.

(c) cover the person with a woolen blanket.

(d) cover the person with a polythene sheet.

Ans: (c) The passage of oxygen or air is reduced by the woollen blanket, which aids in burning. That is why we should wrap the blanket around the person.

8. The substance expected to have the highest ignition temperature out of the following is

(a) kerosene 

(b) petrol 

(c) coal

(d) alcohol

Ans: (c) The lowest temperature at which a substance can catch fire is called the ignition temperature. The lowest ignition temperature is seen in coal.

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.

Ans: (a) The lowest temperature at which a substance can catch fire is called the ignition temperature. Inflammable compounds are difficult to ignite.

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

Ans: (c) Forest fires can occur for a variety of reasons, but they are not caused by tree cutting.

11. The calorific value of a fuel is expressed in a unit called

(a) kilojoule per litre 

(b) kilogram per milliliter 

(c) kilojoule per gram

(d) kilojoule per kilogram

Ans: (d) The calorie value per kilogramme is calculated in kilojoules.

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 environmentally friendly.

(d) it catches fire easily. 

Ans: (b) Due to its abundance, wood is readily available in communities at a moderate cost. That is why it is dominantly used in communities.

13. Which among the following is considered as the cleanest fuel?

(a) cow dung cake 

(b) petrol 

(c) kerosene

(d) hydrogen gas

Ans: (d) In comparison to other fuels, hydrogen emits no hazardous emissions. As a result, it is regarded as 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.

Ans: (c) After burning, a good fuel does not pollute the environment or leave any pollution-causing residue.

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 vessels.

Ans: (c) The vessel became darkened as a result of incorrect fuel combustion.

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.

Ans: 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 __________, the person is covered with a __________ to extinguish fire.

Ans: When the clothes of a person catch fire , the person is covered with a blanket to extinguish fire.

(c) The __________ temperature at which a substance catches fire is called its __________ temperature. 

Ans: The lowest temperature at which a substance catches fire is called its ignition temperature.

(d) The substances which have a very ________________ _ ignition temperature and can easily catch fire with a flame are called __________ substances.

Ans: The substances which have a very low ignition temperature and can easily catch fire with a flame are called combustion substances

(e) The substances which vapourise during __________, give flame.

Ans: The substances which vaporized 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.

Ans: Diesel

(b) Slags is a non-combustible material.

Ans: Glass

(c) Chittsmack does not burn by itself.

Ans: Matchstick

(d) Some substances on combustion produce thea and mafel.

Ans: heat, flame

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

Ans: Calorific

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 lit for a longer time and why?

Ans: The candle will burn for a longer duration if the jar is filled with oxygen gas.

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.

Ans: Anu should boil the water at the flame's hottest zone, which is the non-luminous zone or the flame's outermost zone. 

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

Ans: CNG (Compressed Natural Gas) is a far superior fuel than any other now available. It produces no smoke and emits no toxic gases that are hazardous to the environment. As a result, CNG is displacing conventional fuels such as gasoline.

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.

Ans: 

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.

Ans: True. Air is necessary for combustion

(b) Magnesium is a non-combustible metal.

Ans: False, because magnesium oxide is formed when it reacts with oxygen in the air.

(c) Carbon dioxide is an excellent fire extinguisher.

Ans: True

(d) Calorific value of wood is higher than that of coal.

Ans: False, in comparison to wood, coal has a larger calorific value.

23. Match the items of Column A with the items of Column B.

Ans:

24. Match the following for the flame of a candle.

Ans:

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?

Ans: The iron wire flashes and turns red. It does not emit 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 __________.

Ans: A chemical process in which a substance reacts with oxygen to give off heat is called Combustion 

(b) Wood, paper, CNG is __________ substances.

Ans: Wood, paper, CNG is Combustible substances

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

Ans: The lowest temperature at which a substance catches fire is called its Ignition temperature

(d) Ignition temperature of __________ is lower than that of Wood.

Ans:  Ignition temperature of Petrol  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.

Ans: 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 __________. 

Ans: 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?

Ans: When Angeethi is burned, various hazardous chemicals are released, including carbon dioxide and carbon monoxide. The flow of oxygen is also reduced as a result of this. It causes respiratory problems and can even kill someone who is sleeping. As a result, keeping the door open is recommended to allow for proper air and oxygen movement.

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. 

Ans: False. It’s a chemical process.

(b) Water is the best extinguisher for fires involving electrical equipment.

Ans: False, CO2 (Carbon dioxide) is the best extinguisher.

(c) Alcohol, CNG and LPG are inflammable substances.

Ans: True

(d) Increased concentration of nitrogen in air is believed to cause global warming.

Ans: False, increase in the CO2 in the air is the cause of global warming.

(e) Greater the calorific value the better is the fuel.

Ans: True

(f) Middle zone is the hottest zone of a flame.

Ans: False, Any flame’s outer zone is the hottest zone.

(g) The substances which vapourise during burning, give flame.

Ans: True

29. Cracker on ignition produces sound. Why? 

Ans: When crackers are lit, they emit sound, heat, and light. Explosion is the term for this process. Explosion is a chemical process that occurs when a mixture of gases is present.

30. What do you understand by fuel efficiency?

Ans: The quantity of energy produced by burning 1 kilogramme of full fuel is referred to as fuel efficiency. The caloric value determines this. This is measured in kilojoules per kilogramme.

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?

Ans: Because the watch glass holding fuel has an extremely low ignition temperature, it will burn instantly. Because petrol is a combustible material, it rapidly catches 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?

Ans: Water is heavier than oil. When water is poured over burning oil, it settles to the bottom of the vessel. Pouring dirt or sand, on the other hand, may aid in the extinguishment of the fire by reducing the passage of air. The particles will prevent air from passing through the oil, putting an end to the fire. This occurs when the flow of oxygen is interrupted. And there is no way to burn a fire without oxygen.

33. What are the three essential requirements to produce fire? How a fire extinguisher is useful for controlling the fire.

Ans: Below the essential components to produce fire are mentioned-

(i) Fuel- To give combustion

(ii) Air- Most essential for let the fire burn

(iii) Heat- To provide ignition temperature

Carbon dioxide is contained in fire extinguishers, which helps to restrict the flow of air between the fuel and the air. Because CO2 is heavier than oxygen, it falls between the air and oxygen, assisting in the management of the fire. 

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

Ans: We can use coal and wood as solid fuels. Coal is used in factories and industries, whereas wood is used primarily in residences. Liquid fuels include kerosene and gasoline. Kerosene is primarily used in stoves for cooking, while gasoline is used in automobiles. Gaseous fuels such as CNG and LPG are classified as such. Vehicles run by compressed natural gas (CNG). LPG is also used in automobiles and residences.

35. The calorific values of petrol and CNG are 45000 kJ/kg and 50,000 kJ/kg, respectively. If you have a vehicle which can run on petrol as well as CNG, which fuel will you prefer and why?

Ans: Compressed natural gas has a higher calorific value than gasoline. As a result, CNG will be a suitable option. In addition, it generates more heat energy than gasoline. It emits fewer pollutants and is less expensive than gasoline.

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

Ans: The following factors can be used to discourage the use of wood as a fuel: I It has a high ignition value, making combustion difficult.

(ii) Wood burning emits a variety of hazardous gases, including carbon monoxide and carbon dioxide. Excessive amounts of both can kill a person and contribute to global warming.

(iii) We'd have to cut down trees to utilize wood as a fuel. Trees are necessary for all living things. These are supplied with a variety of natural compounds that aid human survival.

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

Ans: The following factors can be considered: 

(i) When forest grass becomes extremely dry, high temperatures encourage it to burn, resulting in forest fires.

(ii) The presence of lightning in the sky can also be a factor.

(iii) When villagers use fire to scare away animals, the dry grass is burned, resulting in forest fires.

(iv) Friction between bamboo generates heat, which causes the bamboo to burn, resulting in a forest fire.

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 ____ (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)

Ans: Across 

1-Phosphorus 

3- Ignition 

5- Water

Down 

1-Combustion 

4- Fuel 

6- Coal