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Question 1

List conditions under which combustion can take place.

Conditions under which combustion take places:-
(i) Some Fuel is required for combustion.During combustion a fuel substance reacts with oxygen to give off heat.The fuel may be soild, liquid or gas.
(ii) Oxygen in air is essential for combustion. Combustion can not take place in the absence of air (oxygen).
(iii) For a combustion, an inflamabe substance must be heated to its ignition tempetatur. Ignition temperature is the lowest temperature at which a combustible substance catches fire.

Question 3

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

CNG is cheap, readily available and highly combustible. It has high calorific value. It does not produce gases or residues when used in automobiles, so it reduced the pollution in our cities.


Question 4

Compare LPG and wood as fuels.




1.no residue after burning 1.leave a lot of ash on burning
2.can be transport through pipe lines, and cylinder 2.cannot be transport easily like LPG
3.burn easily 3.catch fire with more difficulty
4.low ignition temperature 4.high ignition temperature
5.no smoke on burning 5.burn with smoke

Question 5

Give reason

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 wraped around an aluminium pipe does not.

a. Water is not used to control the fire involving electric equipment because it is a good conductor of electricity.

b. LPG is better domestic fuel than wood because it neither produce gases nor residues that pollute the environment.

c. The paper by itself catches fire easily because its ignition temperature is low, while a piece of paper wraped around an Aluminium pipe does not catches fire, because its ignition temperature rises.

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.

The calorific value of fuel expressed in a unit called kilojoules per kg (kJ kg).


Question 8

Explain how is able to control fires.

When is released by the reaction of sulphuric acid and sodium bicarbonate solution comes out in form of foam with a stream of liquid water at high pressure. Water puts off the fire by lowering the temperature of the combustible material below its ignition temperature and carbondioxide cuts off the supply of air to the combustible substance, and the fire is controlled.


Question 9

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

To burn a heap of green leaves is difficult, because its ignition temperature is high, but dry leaves catch fire easily as its ignition temperature switches to low.


Question 10

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

A goldsmith uses the outermost zone of a flame for melting gold and silver because it is the hottest zone of the flame (temperature » 800° C) and is non-luminous in nature.


Question 11

In an experiment 4.5 kg of a fuel was completely burnt. The heat produced was measure dot be 180,000 kJ. Calculate the calorific value of the fuels.

The calorific value of the fuel:

Calorific value = kJ / kg

= 18,0000 / 4.5 kJ / kg

= 40000 kJ/kg

Question 12

Can the process of rusting be called combustion? Discuss.

The process of rusting cannot be called as combustion because neither release of energy nor heat and light are produced during it, while in combustion – release of energy takes place with heat and light.


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?

Ramesh’s water will get heated in a shorter time.

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