Lakhmir Singh & Manjit Kaur: Combustion and Flame- 2 | Lakhmir Singh & Manjit Kaur Solutions: Class 8 Science PDF Download

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

Water is conductive in nature and hence the electricity in the equipment could reach the extinguisher (the person dousing the fire using water) and can electrocute him/her.

Q.32. How is the fire caused by electricity extinguished? 

To put an electrical fire out, we need to unplug or de-energise the equipment on fire. That will eliminate the electricity that is causing the fire. If one cannot unplug what is burning there is a special type of fire extinguisher.

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

Carbon dioxide is the best fire extinguisher to put out fire caused by inflammable materials like oil and petrol and electrical equipments. Carbon dioxide is heavier than air and it covers the fire and cuts off the supply of oxygen and puts out the fire. Carbon dioxide is stored at high pressure as liquid in cylinders.

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

Q.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. The water vapour produced by the action of heat of fire on water surround the burning material and help in cutting off the supply of air.

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

When fire brigade arrives to the rescue, the fire man throws a strong stream of water on the building on fire, the burning material get cooled down to below their ignition temperature and fire is extinguished. The water vapour produced by the action of heat of fire on water surround the burning material and help in cutting off the supply of air. Fire-men extinguishes the fire by throwing water under pressure on the burning things.

(B) Describe one method of putting out 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. . The water vapour produced by the action of heat of fire on water surround the burning material and help in cutting off the supply of air.

Q.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 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.

Q.38.
(A) What is a flame? What type of substance ,on burning, give a flame? 

Flame is the zone of combustion of a combustible substance. Substances which vapourise during burning produce flames. Eg:- kerosene, wax etc.

(B) What is the difference between the burning of a candle and the burning of a fuel like coal. 

Substances which vapourise during burning, produce flames, such as wax in the candle, Kerosene. Substances which do not vapourise during burning, do not produce flames. Example- coal and charcoal.

Q.39. How does pouring water extinguished a fire. 

Water extinguishes heat by cooling down the burning substance. When water is thrown on substances, it gets cooled below its ignition temperature and also stops burning. The water vapour produced by the action of heat of fire on water surround the burning material and help in cutting off the supply of air.

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

Carbon dioxide is the best fire extinguisher to put out fire caused by inflammable materials like oil and petrol and electrical equipments. Carbon dioxide is heavier than air and it covers the fire and cuts off the supply of oxygen and puts out the fire. Carbon dioxide is stored at high pressure as liquid in cylinders. When released from the cylinder, carbon dioxide expands enormously in volume and cools down. Carbon dioxide not only forms a blanket around the burning substance, it also cools down the burning substance.

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

If the clothes of a person catch fire, the person 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.

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

(a) Solid Fuels: Wood, charcoal, coal, coke, cow-dung cakes.
(b) Liquid Fuels: Kerosene, petrol, diesel, alcohol.
(c) Gaseous fuels: Coal gas, natural gas

Q.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 has three main zones. They are:

• Outer zone
• Middle zone
• Inner zone

In the inner zone there are no burnt vapours of the fuel due to which the colour is black and is least hot(or coldest) part.

Q.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 so that complete combustion of fuel takes place and the temperature of the flame increases. He blow the outermost zone of a flame with a metallic blow pipe for melting gold and silver because this is the hottest part of the flame.

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

(a) In the middle zone partial combustion of the fuel takes place, so the colour of the flame is yellow and it is moderately hot part of the flame. It is the luminous part of the flame.
(b) In the outer zone complete combustion of the fuel takes place, so colour of the flame is blue and it is the hottest part of the flame. It is the non luminous part of the flame.

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

The use of petrol and diesel as fuels in automobiles is being replaced by CNG because

• It produces very small amount of harmful gases.
• It burns without smoke so it is a clean fuel.

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

Disadvantages of burning wood as fuel are as follow:

• Burning of wood produces a lot of smoke which is very harmful for human being. It causes respiratory diseases such as bronchitis and asthma.
• When trees are cut down to obtain wood for use as fuel, then all the useful substances which are obtained from trees are lost.
• Deforestation is very harmful for environment.

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

LPG is a better domestic fuel than wood due to:

• LPG has higher calorific value, so it produces much more heat on burning than an equal mass of wood.
• It burns without leaving behind any smoke.
• It burns completely without leaving behind any solid residue but wood leaves behind a lot of ash on burning.

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

When a burning candle is covered with an inverted gas jar, the oxygen supply gets cut-off. Oxygen is necessary for combustion to take place. It is a supporter of combustion. Hence the candle gets extinguished after sometime.

Q.50. It is difficult to burn a heap of green leaves but dry leaves catch fire easily. Explain.

It is difficult to burn a heap of green leaves but dry leaves catch fire easily. The green leaves contain a lot of water. This water does not allow the green leaves to get heated to their ignition temperature and makes burning of green leaves difficult. Dry leaves do not contain water, they get heated to their ignition temperature easily and hence catches fire easily.

Q.51. 
(A) What are combustible substances? Name three combustible substances. 

Those substances which can burn are called combustible substances. For Ex: Cloth, straw, cooking gas, kerosene oil, wood, coal, charcoal etc. A combustible substance is called as fuel.

(B) What are non-combustible substance? Name three non-combustible substances. 

Those substances which do not burn are called non-combustible substances. For Ex: Stone, cement, glass, bricks, soil, sand, water, iron nails, copper objects.

Q.52.
(A) What is meant by ‘combustion’? Explain with an example.

A chemical process in which a substance reacts with the oxygen to give heat and light is called combustion. The light which is given off during combustion can be in the form of flame or as a glow. The substance which undergoes combustion is said to be combustible. It is called fuel.

(B) What are the conditions necessary for combustion to take place.

There are three conditions which are necessary for combustion to take place:

• Presence of combustible substance.
• Presence of supporter of combustion.
• Heating the combustible substance to its ignition temperature.

Q.53.
(A) Make a labeled diagram of a candle flame.

(B) What makes the middle zone of a candle flame luminous(or light–giving)?

In the middle zone, partial combustion of the fuel takes place, so the colour of the flame is yellow and it is moderately hot part of the flame. It is the luminous part of the flame. It is given in the following picture:

Q.54. What is global warming? Name the gas whose increasing percentage in air is leading to global warming. State a harmful effect which can be caused by global warming.

Global warming is the rise in temperature of earth atmosphere caused by the excessive amount of carbon dioxide in the air. Due to rise in the temperature of atmosphere, the ice in polar regions will melt very fast, producing a lot of water. Burning of fuels releases carbon dioxide in air in the environment. Increased percentage of carbon dioxide in air is causing global warming. Carbon dioxide gas in the air trap sun’s heat rays by producing green house effect. Rise in water may cause a rise in the sea level leading to floods in coastal areas. The low-lying areas may be completely submerged under water leading to loss of life and property.

Q.55. Explain how, burning if fuels such as coal, petrol and diesel leads to acid rain. How is acid rain harmful?

Burning of coal, petrol and diesel produces sulphur dioxide which goes into the air. It is an extremely suffocating and corrosive gas. It may damage our lungs. The burning of petrol and diesel in the engines of vehicles also releases nitrogen oxides into the air. These oxides produced by the burning of fuels dissolve in rain water and form acids. The rain water containing acid is called acid rain. It is harmful for crops, soil and damages buildings.