Saturday, August 11, 2012

CBSE 8th Solved Question Ch: Combustion and Flame Extra Mark Gainer

Class 8th solved question Combustion and Flame
1. Chose the substances which will produce flame on burning and why?  Charcoal, molten wax, paper, coal, mustard oil.
Ans: molten wax and mustard oil.
2. Draw a well labeled diagram for the flame of a candle and answer the following
(a) Which zone is luminous and why ?
(b) Why is the innermost zone grey in colour and the coldest zone ?
(c) Which zone is called the zone of complete combustion and why?
Ans: (a) inner most zone is luminous due to partial combution . The unburnt carbon particles impart a pale yellow colour to the flame.
(b) The innermost zone is grey in colour and the coldest zone because It contains unburnt gas particles. No combustion takes place here as no oxygen is available.
(c)Outer most zone is called the zone of complete combustion Here, carbon and hydrogen are completely oxidized to Carbon dioxide and water vapour.
3. Calorific value of wood is approx.17000-22000kj/kg .what does it mean?
Ans: when 1 kg  fue (wood)  is burnt completely in oxygen produce approx.17000-22000kj heat energy.
4. Why is it dangerous to burn coal in a closed room?
An: When coal burn  in a closed room produce toxic fumes or smoke or other harmful product .
5. Write any four characteristics of good fuel .
Ans: he main characteristics of a good fuel are :-
i) Is readily available and cheap. ii) Does not leave behind any undesirable substances.
iii) Is easy to store and transport.  iv) Burns at a moderate rate.  
 v) Produces a large amount of heat.    vi)  Does not cause pollution.
6. Name the gases which lead to formation of acid rain?
Ans: Oxides of sulphur and nitrogen dissolve in rain water and form acids. Such rain is called Acid Rain. It is very harmful for crops, buildings and soil.
7. In an experiment 10 Kg of a fuel produced 200,000 KJ 0f heat . Calculate the calorific value of the fuel
Ans: The calorific value of a fuel :- is the amount of heat energy produced on complete combustion of 1 kg of a fuel. The calorific valve of a fuel is expressed in kilo joule per kg.
The calorific value of the fuel = 200,000 KJ/10 Kg =200,00 KJ/kg
8. How does CO2 help in fire fighting?
Ans: 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.
9. What are the drawbacks of using solid fuel?
Ans: The drawbacks of solid fuels are as follows:
1. They have a high ignition temperature.
2. They produce a large amount of residue (soot, ash) after combustion.
3. Their calorific value is low.
10. What are the harmful effects on the environment due to burning of fuels?
Ans: i) Fuels like wood, coal, petroleum release unburnt carbon particles which cause respiratory diseases like asthma.
ii) Incomplete combustion of fuels release carbon monoxide gas which is a very poisonous gas which can cause death.
iii) Burning of most fuels release carbon dioxide gas which causes rise in the temperature of the atmosphere. This is called global warming. It causes melting of polar ice, rise in sea level and flooding of coastal areas.


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