NCERT solutions for class 8 Science Chapter 6 Combustion And Flame
Solution for Exercise
Q.1 List conditions under which combustion can take place.
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) Burning of wood and coal causes __pollution__ of air.
(b) A liquid fuel, used in homes is _LPG_.
(c) Fuel must be heated to its __ignition temperature__before it starts burning.
(d) Fire produced by oil cannot be controlled by __water__.
Q.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 as it is a quality fuel and has some benefits:
(a) It gives out less carbon dioxide gas, carbon monoxide gas, sulphur dioxide and nitrogen dioxide, which is beneficial as they play crucial role in global warming and acid rain.
(b) It leaves behind no residue after its combustion.
Q.4 Compare LPG and wood as fuels.
Answer :
LPG
(i) It does not cause pollution on combustion.
(ii) No smoke is produced.
(iii) It is a liquid fuel.
(iv) It has more calorific value (55000 kJ/kg).
(v) It can be easily transported, as it is stored in cylinders.
Wood
(i) It pollutes air on its combustion.
(ii) It produces smoke.
(iii) It is a solid fuel.
(iv) It has less calorific value (17000 kJ/kg).
(v) It can’t be transported easily like LPG fuels.
Q.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 is a good conductor of electricity.
If added to an electrical fire, the water would just spread the electricity further.
The person dousing the fire might get an electric shock
b) LPG being a cleaner fuel than wood doesn’t release smoke and other pollutants.
Wood, on the other hand, releases a lot of smoke and fumes polluting the atmosphere causing pollution and leading to respiratory diseases.
Hence, LPG is a better domestic fuel than wood.
c) The paper by itself catches fire easily because of its low ignition temperature.
The piece of paper wrapped around an aluminium pipe doesn’t catch fire because aluminium is a good conductor of electricity.
While the paper wrapped around an aluminium pipe results in an increase in ignition temperature. So, there is a transfer of heat from paper to the aluminium pipe. Hence it doesn’t catch fire.
Q.8 Explain how CO2 is able to control fires.
Answer :
CO2 is a non-combustible gas and extinguishes fire in two ways:
(i) It is heavier than oxygen and it covers the fire like a blanket and cuts off the contact between oxygen and fuel.
(ii) In cylinders, CO2 is kept in the liquid form. When released, it expands enormously. This brings down the temperature of the fuel, which helps in controlling the fire.
Q.9 It is difficult to burn a heap of green leaves but dry leaves catch fire easily. Explain.
Answer :
A heap of green leaves contains a lot of moisture in it, hence its ignition temperature is high. Therefore, it does not catch fire easily.
But dry leaves have no moisture content in it, hence its ignition temperature is low. Therefore, it catches fire easily.
Q.10 Which zone of a flame does a goldsmith use for melting gold and silver and why?
Q.12 Can the process of rusting be called combustion? Discuss.
Answer :
No, because rusting is an exothermic process as heat is liberated during rusting. On the other hand, combustion is a chemical process in which a substance reacts with oxygen to release energy in the form of heat or light.
Q.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 :
The water placed in the outermost part of the flame will be heated in a short time since it is a non-luminous flame and is regarded as the hottest part of the flame. So Ramesh’s beaker will be heated first. However, Abida who placed the beaker in the luminous flame (yellow flame) is comparatively less hot.
