RBSE Solutions for Class 8 Science Chapter 6 Combustion and Flame

Rajasthan Board RBSE Solutions for Class 8 Science Chapter 6 Combustion and Flame  Textbook Exercise Questions and Answers.

RBSE Class 8 Science Solutions Chapter 6 Combustion and Flame

RBSE Class 8 Science Combustion and Flame InText Questions and Answers

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Question 1. 
We have read that the Sun produces its own heat and light. Is it also some kind of combustion?
Answer:
In the sun, heat and light are produced by nuclear reactions. So it is also some kind of combustion.

RBSE Class 8 Science Combustion and Flame Textbook Questions and Answers     

Question 1. 
List conditions under which combustion can take place.
Answer:
Conditions of combustion:

1. The presence of inflammable substance is essential.
2. Presence of oxygen is essential.
3. Heating of inflammable substance upto its ignition temperature.

Question 2. 
Fill in the blanks:
(a) Burning of wood and coal ................ causes of air.
(b) A liquid fuel, used in homes is ................    
(c) Fuel must be heated to its  ................ , ................ before it starts burning.
(d) Fire produced by oil cannot be controlled by ................ 
Answer:
(a) pollution 
(b) kerosene 
(c) ignition temperature 
(d) water.

Question 3. 
Explain how the use of CNG in automobiles has reduced pollution in our cities?
Answer:
The use of CNG in automobiles has reduced pollution in our cities because:

1. CNG is a cleaner fuel.
2. CNG releases lesser amount of oxides of sulphur and nitrogen. So acid rain do not occurs, due to this reason there is no harmful effect on crops, buildings and soil.
3. There is complete combustion of CNG in air and no harmful gases are released.

Question 4. 
Compare LPG and wood as fuels. 
Answer:
Comparison of LPG and wood as fuels:

Question 5. 
Give reasons:
(a) Water is not used to control fires involving electrical equipment.
(b) LPG is a better domestic fuel than wood.
(c) Paper by itself catches fire easily whereas a piece of paper wrapped around an aluminium pipe does not.
Answer:
(a) Water may conduct electricity and harm those trying to do put off the fire. So water is not used to control fires involving electrical equipment.

(b) 

1. The calorific value of LPG in comparison to wood is high i.e. more heat is produced on burning of LPG while less heat is produced on burning same quantity of wood.
2. LPG bums completely and no residues are left while burning of wood left large amount of ash. So pollution is not created on burning of LPG while pollution is created by the burning of wood.

(c) Aluminium is good conductor of heat. Piece of paper wrapped around an aluminium pipe does not catche fire, because heat is transferred to aluminium and the tempereature of paper does not reach up to ignition temperature.

Question 6. 
Make a labelled diagram of a candle flame.
Answer:

Question 7. 
Name the unit in which the calorific value of a fuel is expressed.
Answer:
The calorific value of a fuel is expressed in kilo joule per kilogram.

Question 8. 
Explain how CO₂ is able to control fires.
Answer:
CO₂, being heavier than oxygen, cover the fire like a blanket, since the contact between the fuel and oxygen is cut off, the fire is controlled.

Question 9. 
It is difficult to burn a heap of green leaves but dry leaves catch fire easily. Explain.
Answer:
Green leaves have moisture so temperature of these leaves decreases and ignition temperature increases so it is difficult to bum heap of green leaves but the ignition temperature of dry leaves is low so they catch fire easily.

Question 10. 
Which zone of a flame does a goldsmith use for melting gold and silver and why?
Answer:
Goldsmiths blow the outer most zone of a flame with a metallic blow pipe for melting gold and silver because this is die hottest part of flame.

Question 11. 
In an experiment 4.5 kg. of a fuel was completely burnt. The heat produced was measured to be 1,80,000 kJ. Calculate the calorific value of the fuel.
Answer:
The heat produced by the complete combustion of 4.5 kg.
fuel = 1,80,000 kJ. So the heat produced by the complete combustion of 1 kg.
fuel = \(\frac{1,80,000}{4.5} \) = 40,000
So the calorific value of fuel = 40,000 kJ/kg. 

Question 12. 
Can the process of rusting be called combustion? .Discuss.
Answer:
The process of rusting can not be called combustion because during rusting heat and light are not produced. Besides this, iron is not a combustible substance.

Question 13. 
Abida and Ramesh were doing an experiment in which water was to be heated in a beaker. Abida kept the beaker near the wick in the yellow part of the candle flame. Ramesh kept the beaker in the outermost part of the flame. Whose water will get heated in a shorter time?
Answer:
Ramesh's beaker will get heated water in shorter time because the Qutermost part of the flame is hottest.