Combustion and Flame
Science Class Eight
NCERT Exercise Solution
Question (1) List the conditions under which combustion can take place.
Answer Conditions under which combustion take place
(a) Availability of fuel
(b) Supply of oxygen
(c) Reaching the combustion to minimum ignition temperature.
Question (2) Fill in the blanks
(a) Bruning of wood and coal can causes _________ of air.
Answer (a) Bruning of wood and coal can causes pollution of air.
(b) A liquid fuel, used in homes is ________.
Answer A liquid fuel, used in homes is LPG (Liquid Petroleum Gas).
(c) Fuel must be heated to its ______ ______ before it starts.
Answer Fuel must be heated to its ignition temperature before it starts.
(d) Fire produced by oil cannot be controlled by ______.
Answer Fire produced by oil cannot be controlled by water.
Question (3) Explain how the use of CNG in automobiles has reduced pollution in our cities.
Answer
CNG releases less pollutant when burnt. CNG is cheaper fuel than other fossil fuels such as petrol and diesel. CNG burns directly in engine like petrol and diesel. Since CNG creates less pollution thus it is said that CNG is a cleaner fuel.
Thus by using CNG in automobiles has reduced pollution in our cities.
Question (4) Compare LPG and wood as fuels.
Answer
| Comparision of LPG & Wood as fuel | ||
|---|---|---|
| LPG | Wood | |
| LPG being in liquid form easily stored in compressed cylinders and can be transported to households using pipelines | Wood is solid and hence cannot be compressed into cylinders and cannot be transported using pipe lines | |
| Use of LPG do not associated with deforestation | Use of wood as fuel resulting in deforestation | |
| LPG on burning releases no smoke | Wood on burning releases too much smoke | |
| LPG releases less pollutant | Wood on burning releases too much pollutant | |
| LPG has more calorific value | Wood has very less calorific value than that of LPG | |
| Use of LPG as fuel is very easy | Use of wood as fuel is very difficult | |
| LPG is cleaner fuel | Wood is not a cleaner fuel as it creates too much pollution | |
Question (5) Give reasons.
(a) Water is not used to control fires involving electrical equipment.
Answer
Water is good conductor of electricity. Thus if water if poured on fire involving electricity it may be proved hazardrous for people trapped in fire as well as fire brigade personnel.
Thus, water is not used to control fires involving electrical equipments.
(b) LPG is a better dometic fuel than wood.
Answer
It is easy to use LPG compare to wood as fuel. LPG creates less pollution when burning while wood on burning releases too much smoke in the form of carbon dioxide, carbon monoxide and water vapour. LPG can be stored in compressed cylinders and transported through pipe line also whereas wood cannot. Uses of wood as fuel lead to deforestation whereas LPG does not. LPG has more calorific value than that of wood.
Thus, 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
Paper has lower ignition temperature and thus caches fire easily. But when paper is wrapped around an aluminium pipe, heat supplied to paper is transferred to the aluminium pipe as aluminium is very good conductor of heat.
These are the cause that paper by itself catches fire easily whereas a piece of paper wrapped around an aluminium pipe does not.
Question (6) Make a labeled diagram of a candle flame.
Question (7) Name the unit in which the calorific value of a fuel is expressed.
Answer
Unit of calorific value is kilojue per kg (kJ/kg).
Question (8) Explain how CO2 is able to control fires.
Answer
CO2 is heavier than air thus when it is spread over fire it covers fire. CO2 does not support combustion thus after covering fire it cut the supply of oxygen to the combustible. On the other hand CO2 lowers down the temperature of combustible also.
Thus, spreading of CO2 is able to control fires.
Question (9) It is difficult to burn a heap of green leaves but dry leaves catch fire easily. Explain.
Answer
Green leaves contains lot of water but dry leaves do not. When heat is introduced to heap of green leaves it takes too much time to reach the minimum ignition temperature because of presence of lot of moisture. But dry leaves do not have moisture or very less moisture thus when heat is supplied to heap of dry leaves, it reaches to ignition temperature easily and quickly.
Thus, it is difficult to burn a heap of green leaves but dry leaves catches fire easily.
Question (10) Which zone of a flame does a goldsmith use for melting gold and silver and why?
Answer
Goldsmiths use outer zone of a flame for melting gold and silver. Because outer zone of a flame is the hottest zone.
This is the cause that a goldsmith use outer zone of flame for melting gold and silver.
Question (11) In an experiment 4.5 kg of a fuel was completely burnt. The heat produced was measured to be 180,000 kJ. Calculate the calorific value of the fuel.
Solution
Given, total weight of fuel = 4.5 kg
And Head produced = 180,000 kJ
Since, calorific value is measured as kilo jule per kg
i.e. kJ/kg
Thus, total calorific value = heat produced in kJ / weight of fuel
`= (180000 kJ)/(4.5 kg)`
= 40000 kJ/kg
Thus, calorific value = 40000 kJ/kg Answer
Question (12) Can the process of rusting be called combustion? Discuss.
Answer
Rusting of iron is actually oxidation of iron in the presence of moisture or water, whereas combustion is the heating of fuel in the presence of oxygen.
Since, in the rusting of iron heat is not involed, thus rusting cannot be called combustion.
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
Outermost part of a flame is the hottest zone while innermost part of a flame is the zone of unburnt vapour of fuel and least hot zone.
Thus, water kept in the beaker of Ramesh will get heated in shorter time because he kept the beaker in the outermost part of flame which is the hottest part of a flame.
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