# Lakhmir Singh Solutions For Class 8 Science Chapter 6 Combustion And Flame

Lakhmir Singh Solutions for Class 8 Science Chapter 6 Combustion and Flame are provided in this article. Have you ever noticed the difference between combustion and flame? The answer to this question is dealt with in this chapter. This lesson gives us knowledge of the substances which produce flame on burning and the different zones present in the flame. We will also learn about the substances that combust on burning and the various classifications of combustions.

This chapter contains a topic on fire, which briefly describes how to act when a person catches fire, how to act when a building catches fire and measures to extinguish the fire. All queries related to burning, combustion, and fire are explained in this lesson.

## Very Short Answer Type Questions

1. Define the ignition temperature of a substance.

The lowest temperature at which a combustible substance catches fire when heated in the air is called its ignition temperature of a substance.

2. Which of the two has a lower ignition temperature: petrol or kerosene?

Petrol has a lower ignition temperature.

3. Name the most common fire extinguisher.

The most common fire extinguisher is water.

4. Which is the best fire extinguisher for fires involving electrical equipment and inflammable materials like petrol?

The best fire extinguisher for fires involving electrical equipment and inflammable materials like petrol is carbon dioxide.

5. Name one substance which undergoes spontaneous combustion (or burns in air at room temperature).

White phosphorous undergoes spontaneous combustion (or burns in air at room temperature).

6. Name the unit in which the calorific value is expressed.

The calorific value is expressed in energy per unit mass of the substance. Example KJ/Kg

7. Which of the following fuels has the lowest calorific value?

Diesel, Methane, Coal, CNG, Petrol

Coal has the lowest calorific value.

8. Which of the following fuels has the highest calorific value?

Diesel, Methane, CNG, Coal, Petrol

CNG and Methane have the highest calorific value.

9. Name the term which is used to express the efficiency of fuel.

Calorific value is used to express the efficiency of fuel.

10. Name one solid, one liquid and one gas which burns by producing a flame.

Solid: Molten wax

Liquid: Kerosene

Gas: Liquified Petroleum Gas

11. Which of the following does not produces a flame on burning?

Camphor, Charcoal, Kerosene

Charcoal does not produce flame on burning.

12. Name one fuel which burns without producing a flame.

Charcoal burns without producing flame.

13. How many zones are there in a flame?

There are three zones in a flame. Namely, the innermost, middle and outer zone.

14. Which zone of a candle flame is the hottest?

The outermost zone of the candle is the hottest.

15. In a candle flame, what is the colour of: (a) the innermost zone (b) the middle zone and (c) the outer zone?

(a) Dark or black

(b) Yellow

(c) Blue

16. Name any harmful product released by the burning of fuels.

Carbon monoxide is one of the most harmful products released by the burning of fuels.

17. Name the very poisonous gas produced by the incomplete combustion of fuels.

Incomplete combustion of fuels produces poisonous gas called carbon monoxide.

18. Name the fuel which is gradually replacing petrol and diesel in automobiles.

CNG is gradually replacing petrol and diesel in automobiles.

19. Name two substances having low ignition temperature and two having high ignition temperatures.

Substances having low ignition temperature: LPG and Petrol

Substances having high ignition temperature: Coal and wood

20. Fill in the blanks with suitable words

(a) A fuel must be heated to its_________ before it starts burning.

(b)The most common supporter of combustion around us is _________

(c)Fire produced by burning oil cannot be controlled by_________

(d)A liquid fuel used in homes is_________

(e)The amount of heat evolved when 1kg of a fuel is burnt completely is called its_________

(f)The substance which vaporises during, give_________

(g)Burning of wood and coal causes_________ of air.

(a) A fuel must be heated to its ignition temperature before it starts burning.

(b)The most common supporter of combustion around us is air.

(c)Fire produced by burning oil cannot be controlled by water.

(d)A liquid fuel used in homes is kerosene.

(e)The amount of heat evolved when 1kg of a fuel is burnt completely is called its calorific value.

(f)The substance which vaporises during giving flames.

(g)Burning of wood and coal causes pollution of the air.

21. (a) What are fuels? Name any two common fuels.

(b) State any four characteristics of an ideal fuel (or good fuel).

(a) Any substance which, upon combustion, produces a usable amount of energy is known as fuel. For example, fossil fuels, biogas, nuclear energy etc.

(b) Characteristics of an ideal fuel (or good fuel) are:

• Fuel is easily available and cheaper
• Burns easily in the air at a moderate rate
• Releases a large amount of energy
• No harmful substances must be emitted from the fuel
• Eco friendly

22. (a) Define the calorific value of a fuel.

(b) “The calorific value of LPG is 55000KJ/kg”. What does it mean?

(a) The amount of heat energy produced on complete combustion of 1 kg of a fuel is called its calorific value. The calorific value of a fuel is expressed in a unit called kilojoule per kg (kJ/kg).

(b) The calorific value of LPG is 55000KJ/kg means 1 kilogram of LPG is burned completely, and 55000KJ/kg of heat energy is produced.

23. Can you burn a piece of wood by bringing a lighted matchstick near it? Explain.

No, we cannot burn a piece of wood by bringing a lighted matchstick near it. When a lighted matchstick is brought near a piece of wood, it does not start burning because the ignition temperature of the wood is higher than a piece of paper. A matchstick can light a tiny splinter of wood but not a big log of wood. A burning matchstick can produce sufficient heat to reach the ignition temperature of the splinter of wood; therefore, a matchstick can light a splinter of wood directly.

24. Why do you have to use paper or kerosene oil to start a fire in wood or coal?

The ignition temperature of wood or coal is higher, and it requires more heat to start burning. This is the cause that to start a fire in wood or coal, paper or kerosene oil is used as the ignition temperature of paper or kerosene oil is lower than that of wood or coal.

25. What is meant by rapid combustion? Give one example of rapid combustion.

When a substance burns rapidly, with the aid of an external source and produces heat within a short period of time is called rapid combustion. Example: Burning of LPG

26. What is meant by spontaneous combustion? Give one example of spontaneous combustion.

The type of combustion in which material suddenly bursts into flames without the application of any apparent cause is called spontaneous combustion. Eg., Burning of phosphorus.

27. What is meant by explosive combustion (or explosion)? Give one example of explosive combustion (or explosion).

A sudden reaction takes place with the release of heat and light, and the evolution of a large amount of gas takes place it is called an explosion. Eg., firecrackers.

28. How will you show that air is necessary for combustion?

Air helps in combustion. Air contains about twenty-one per cent of oxygen, and this oxygen present in the air helps in the process of combustion. Without oxygen, combustion will not take place.

29. Can the process of rusting be called combustion? Give a reason for your answer.

No, because rusting is an exothermic process as heat is liberated during rusting. On the other hand, combustion is a chemical process in which a substance reacts with oxygen to release energy in the form of heat or light.

30. Why are fires produced by burning oil not extinguished by pouring water?

Water is heavier than oil. So when it is poured on an oil fire, it sinks to the bottom where it evaporates due to the heat of the fire and thus expands rapidly, due to which it may push and splatter fire in all directions. Fire due to oil can be extinguished by using a dry chemical fire extinguisher.

31. Explain why fire caused by electricity should not be extinguished by pouring water.

The most common fire extinguisher is water. But waterworks only when things like wood and paper are on fire. If electrical equipment is on fire, water may conduct electricity and harm those trying to douse the fire. For fires involving electrical equipment and inflammable materials like petrol, carbon dioxide (CO2) is the best extinguisher. CO2, being heavier than oxygen, covers the fire like a blanket. Since the contact between the fuel and oxygen is cut off, the fire is controlled. The added advantage of CO2 is that, in most cases, it does not harm electrical equipment.

32. How is the fire caused by electricity extinguished?

The fire caused by electricity can be extinguished by petrol or by carbon dioxide. For fires involving electrical equipment and inflammable materials like petrol, carbon dioxide (CO2) is the best extinguisher. CO2, being heavier than oxygen, covers the fire like a blanket. Since the contact between the fuel and oxygen is cut off, the fire is controlled. The added advantage of CO2 is that, in most cases, it does not harm electrical equipment.

33. How is the fire produced by burning oil (or petrol) extinguished?

The fires caused by the burning of inflammable materials like oil or petrol are also extinguished by using carbon dioxide fire extinguishers. The carbon dioxide used for extinguishing fire can be stored as a liquid at high pressure in cylinders.

34. A drum full of kerosene catches fire. What is the simplest way to put off this fire?

A small fire, like a drum of kerosene on the fire, can be extinguished by throwing sand or soil over it. When sand is thrown over burning kerosene oil, the sand covers it like a blanket. The sand cuts off the air supply to the burning kerosene oil, due to which the fire gets extinguished.

35. What is the first thing you should do if a fire caused by burning wood or paper?

Water extinguishes heat by cooling down the burning substance, such as wood and paper. When water is thrown on these substances, it gets cooled below its ignition temperature and also stops burning.

36. (a) What does a Fire Brigade do when it arrives at a place where a building is on fire?

(b) Describe one method of putting out a fire caused by burning wood or paper.

(a) When the fire brigade arrives at the rescue, the fireman throws a strong stream of water on the building on fire, the burning material gets cooled down to below its ignition temperature, and the fire is extinguished.

(b) Water extinguishes heat by cooling down the burning substance, such as wood and paper. When water is thrown on these substances, it gets cooled below its ignition temperature and also stops burning.

37. Explain why, we are advised not to sleep in a room having closed doors and windows, with a coal fire burning inside.

Due to the burning of coal, the available oxygen gets depleted, and it leads to the incomplete burning of coal. Incomplete combustion of coal gives carbon monoxide gas. It is a very poisonous gas. It is dangerous to burn coal in a closed room. The carbon monoxide gas produced can kill persons sleeping in that closed room

38. (a) What is a flame? What type of substance, on burning, gives a flame?

(b) What is the difference between the burning of a candle and the burning of a fuel like coal?

(a) A hot glowing body of ignited gas that is generated by something on fire is called flame. Kerosene oil and molten wax are substances that give a flame while burning.

(b) Substances which vapourize during burning produce flames, such as wax in the candle, Kerosene.

Substances which do not vapourize during burning do not produce flames. Example – coal and charcoal.

39. How does pouring water extinguish a fire?

Water lowers the temperature of the burning substance. When the temperature goes down below the ignition temperature of the burning substance, the fire extinguishes. The water here acts as a cooling agent.

40. Explain how carbon dioxide is able to control fires?

Carbon dioxide is heavier than oxygen and covers the fire like a blanket. Since the contact between the fuel and oxygen is cut off, the fire is controlled. The added advantage of CO2 is that, in most cases, it does not harm electrical equipment.

41. If you see a person whose clothes are on fire, how will you extinguish the fire? Give a reason for your answer.

The person whose clothes are on fire should be immediately covered with a blanket. When the burning clothes of a person are covered with a blanket, the supply of air to the burning clothes is cut off, and hence the burning stops.

42. Give two examples each of: (a) solid fuels (b) liquid fuels (c) gaseous fuels

Solid Fuels

1. Coal- Is used to produce electricity through steam engines.
2. Coke- Coke is used as a reducing agent in the extraction of metals.

Liquid Fuels

1. Petrol: Used to run small automobiles like bikes and cars.
2. Kerosene- Used for domestic heating purposes and jet engines as fuel.

Gaseous fuel

1. CNG – CNG is used to run automobiles.
2. Natural Gas – It is used for industrial purposes

43. Name the various zones of a candle flame. Which zone(or part) of a candle flame is the least hot(or coldest)?

A candle flame consists of three different zones. Each zone has different temperatures. Every zone has a different colour, and this will help us understand the temperatures of each zone.

1. The first zone is called the Outer zone
2. The second zone is called the Middle zone
3. The third zone is called as Inner zone

The least hot region of the flame is present innermost. This inner zone is black in colour due to the presence of unburnt wax vapours.

44. Why does a goldsmith blow air into the kerosene lamp flame with a blow pipe?

A goldsmith blows air into the kerosene lamp flame with a blow pipe to ensure that the combustion of fuel takes place and the temperature of the flame increases. The goldsmith mainly uses a non-luminous flame which is termed to be the outermost part of the flame. This part of the flame is used because the outermost flame undergoes complete combustion and is considered as the hottest part of the flame.

45. In which zone of a candle flame: (a) partial combustion of fuel takes place, and (b) complete combustion of fuel take place?

(a) Middle zone

(b) Outer zone

46. Explain how the use of CNG in automobiles has reduced pollution in cities.

CNG plays an important role in reducing pollution among automobiles for the following reasons:

• CNG is comparatively a cleaner fuel.
• CNG can be an alternative to diesel, petrol and propane/LPG.
• It usually contains few undesirable gases than the other fuels mentioned above.
• The combustion of fuels like petroleum causes many unburnt carbon particles along with carbon monoxide, which leads to respiratory diseases.

47. What are the disadvantage of burning wood as fuel?

The burning of fuels like wood as fuel releases unburnt carbon particles in the air which causes respiratory problems. Incomplete combustion of fuels produces a very poisonous gas called carbon monoxide.

48. Give reasons for the following: LPG is a better domestic fuel than wood.

LPG is a better domestic fuel than wood due to the following reasons:

• It doesn’t release smoke and other pollutants
• It is a cleaner fuel
• The fuel efficiency of LPG is more than that of wood.
• The calorific value of LPG is 55000 kJ/kg

49. Explain why, when a burning candle is covered with an inverted gas jar, the candle gets extinguished after some time.