Dav Class 8 science chapter 7 question answer |🔥 Combustion | Q.No.A,B,C,D,E. | Pdf Download |

 Class 8
Chapter 7
Combustion

PDF :- Science Chapter 7

QUESTION AND ANSWERS

A.

1. Higher

2. Slow

3. Innermost

4. Hottest

5. Carbon monoxide

B.

1. True

2. False

3. True

4. False

5. True

C.

1. Iron

2. It's ignition temperature

3. Blue Zone

4. (0.1 H) KJ/Kg

5. Carbon monoxide

D.

1. Combustible material is a substance which on heating in the presence of oxygen catches

fire easily and produces heat and light energy.

eg.Wood,LPG

2. Three condition that are needed for combustion are:

  There should be combustible materials.

  There should be a continuous supply of oxygen.

  The temperature should be above ignition temperature of combustible material.

3. Slow combustion usually occurs when there is an insufficient supply of air.

4. Calorific value of fuel is the amount of heat energy produced on complete burning of one

kilogram of fuel in pure oxygen.

5. Characteristic of an ideal fuel is

  It is cheap and readily available.

  It is easy to store and transport.

  It has high calorific value.

  It has low ignition temperature however it should be above room temperature.

  It does not produce harmful gases or ash.

6. Water is not used in controlling electrical fires because water is a good conductor of

electricity and can cause dangerous electric shock.

E.

1. Diagram of a candle flame showing different zones

Candle Flame 

2. Calorific value of wood equals to 18000 KJ/Kg

Heat energy produced by wood 360,000 kj

Calorific value = Energy/Mass

Mass=360000/18000

So, 20 kg of wood is required to produce 360000 KJ of heat energy.

3. Gaseous fuels are better than liquid or solid fuels as

  They are easier to transport.

  They have low ignition temperature.

  They do not leave harmful gases on burning.

  They produce a very large amount of heat and they do not leave any ash or residue on

combustion.

4. Burning of fuels cause health hazards as:

  Fuels release unburnt carbon particles . They move in air and in winter form smog.

  They result in formation of carbon monoxide .It is a harmful and poisnous gas.

  Some fuels release carbon dioxide in the environment which can raise temperatures of

earth and cause global warming.

  Burning of coal and diesel release sulphur dioxide which mix with water vapour and

come down in rain making it acidic and is the cause of Acid Rain.

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