Combustion And Flame

Exercises 

1. List conditions under which combustion can take place.

Ans:-
Following are the required for combustion :

1. presence of fuel
2. air to supply Oxygen
3. heat to raise the temperature of the fuel beyond its ignition temperature 

2. Fill in the blanks.

(a) Burning of wood and coal causes      pollution      of air.

(b) A liquid fuel, used in homes is      liquefied petroleum gas(LPG)      .

(c) Fuel must be heated to its before it      ignition temperature     starts burning.

(d) Fire produced by oil cannot be controlled by      water     .

3. Explain how the use of CNG in automobiles has reduced pollution in our cities.

Ans:-
CNG is termed as a clean fuel because CNG produces the harmful products in very small amounts. It has replaced the use of petrol, diesels as a fuel for automobiles because these are carbon fuels and combustion of such fuels causes release of unburnt carbon substances like CO (carbon monoxide) which is a very toxic gas and can lead to respiratory diseases, such as asthma.

4. Compare LPG and wood as fuels.

Ans:-

5. Give reasons.

(a) Water is not used to control fires involving electrical equipment.

Ans:-
Water is not used to control fires involving electrical equipment because it is hood conductor of electricity and the person can get electric shock if he tries to extinguish fire using water.

(b) LPG is a better domestic fuel than wood.

Ans:-
LPG is better domestic fuel than wood because of the following reasons :

1. On combustion, doesn't causes air pollution
2. doesn't produces smoke
3. easy to store and transport through cylinders and pipes
4. high calorific value : 55000 kJ/kg

·    (c) Paper by itself catches fire easily whereas a piece of paper wrapped around an aluminium pipe does not.

Ans:-
Paper by itself catches fire easily whereas a piece of paper wrapped around an aluminium pipe does not. This is so because because paper itself is a combustible substance and has low ignition temperature but aluminum is a good conductor of heat, so heat absorbed by the paper is taken away by the aluminum pipe.

6. Make a labelled diagram of a candle flame.

Ans:-

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

Ans:-
The unit in which the calorific value is expressed - kilojoule per kilogram (kJ/kg).

8. Explain how CO2 is able to control fires.

Ans:-
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. It can be stored at high pressure as a liquid in cylinders. When released from the cylinder, CO2 expands enormously in volume and cools down. So, it not only forms a blanket around the fire, it also brings down the temperature of the fuel.The added advantage of CO2 is that in most cases it does not harm the electrical equipment.


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

Ans:-
It is difficult to burn a heap of green leaves but dry leaves catch fire easily because green leaves contains moisture so they has high ignition temperature whereas dry leaves has low ignition temperature due to the absence of moisture.


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

Ans:-
Goldsmiths uses the outermost zone of a flame with a metallic blow-pipe for melting gold and silver. This is so because this part is the hottest part of the flame and results in complete combustion.


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.

Ans:-
heat produced = 180,000 kJ
mass of fuel = 4.5 kg
since calorific value  = heat produced / mass of fuel
                                     = 180,000 kJ / 4.5 kg
                                     =   40,000 kJ/kg   


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

Ans:-
No, the process of rusting cannot be called as combustion because of:

1. though it reacts with oxygen but doesn't liberates heat.
2. rusting can take place in room temperature but combustion happens below ignition temperature.

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?

Ans:-
Ramesh's water will get heated in a shorter time because this part is the hottest part of the flame.