Solutions 6. Combustion And Flame - Text Book Exercise | Class 8 Science - Toppers Study

Exercise

Q1. List conditions under which combustion can take place. 

Answer :

Conditions under which combustion can take place are as follows : 

(i) Air (oxygen), (ii) the ignition temperature should reach soon, and (iii) a source of inflammable substance.

Q2. 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) LPG, (c) ignition temperature, (d) water. 

Q3. 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 CNG does not produce any poisonous gas/es on buring. That is why pollution in our cities is reduced.  

Q4. Compare LPG and wood as fuels.

Answer :

LPG burns easily and produces move heat in comparison to wood. Besides, it is a clan fual i.e., it does not produce fume and ashes as wood do.  

Q5. 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 not used to control fire produced by electrical ecuipment because water is a conductor of electricity and may result in electrocution or electric shock.

(b) LPG is asubstance which is readily available. It is cheaper and burns easily in air at moderate rate. It produces a large amount of heat and does not leave behind any undesirable substance. In case of wood, it does not burn that easily and produces a large amount of smoke which is very harmful for human beings. That is why, wood is not a better fuel than LPG.

(c) Paper catches fire easily, but when it is wrapped around an aluminium pipe and heat is suppiled then in presence of aluminium pipe, the ignition temperature does not meet, because heat given to it is passed to aluminium pipewhich lowers the temperture of the paper wrapped around it. That is why paper does not catch fire.

Q6. Make a labelled diagram of a candle flame.

Answer : 

Fig. 6.1 A candle flame 

Q7. Name the unit in which the calorific value of a fuel is expressed.

​Answer : 

The calorific value of a fuel is expressed in kilojoules per kg (kJ/kg).

Q8. Explain how CO₂ is able to control .

​Answer : 

CO₂ being heavier than oxygen, covers the fire like a blanket. Since the contact between the fuel and oxygen is cut off, the fire is controlled. The added advantage of CO₂ is that in most cases it does not harm the electrical equipment.  

Q9. It is difficult to burn a heap of green leaves but dry leaves catch fire easily. Explain.

​Answer :

Green leaves contain lot of water. So, when we try to burn green leaves, water contained in  the leaves cools the combustible materials (leavss), so thaqt its temperature is brought below its ignition temperture. This prevents the burning of green leaves. In case of dry leaves, they do not contain any water. So when buring process starts, its temperature is raised drastically above its ignition temperature and the leaves catch fire easily.    

Q10. Which zone of a flame does a goldsmith use for melting gold and silver and why?

​Answer :

 The goldsmith uses the outermost zone of a flame with a metallic blow-pipe for melting gold and silver. The flame in the outermost zone has the highest temperature sufficient to melt the gold and silver.    

Q11. 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.

​Answer :

Calorific value of fuels

= kilojoules/kg =  (Total heat produced/Total mass burent)

Here, the mass of fuel =4.5 kg 

The heat produced =180,000 kJ.

Calorific value of fuel =180,000 kJ/ 4.5 kg

= 40,000 kJ per kg.

Q12. Can the process of rusting be called combustion? Discuss.

​Answer : 

Yes, process of rusting can be called combution, in fact, slow combution, because rusting also takes place in the air (O₂) in presence of humidity in the atmosphere. 

Q13. 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 of Ramesh,s beaker will get heated with a shorter time because the outermost part of the flame the hottest.