Combustion And Flame, Class 8th Notes

THESE NOTES ARE PREPARED ACCORDING TO JKBOSE SYLLABI IN SIMPLIFIED AND CONCISE PATTERN

Exercises

Q. List the conditions in which combustion can take place?

Ans: Combustion can take place under following conditions;

1. Presence of oxygen

2. Presence of Fuel

3. Ignition temperature.

Q. Fill in the blanks:

a. Burning of wood and coal causes air .......

Ans: Pollution

b. A liquid fuel used in homes is .......

Ans: Kerosene

c. Fuel must be heated to its ...... before it starts burning.

Ans: Ignition temperature.

d. Fire produced by oil cannot be controlled by......

Ans: Water.

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

Ans: The Compressed natural gas is a clean fuel. Unlike petrol, It does not produce Carbon monoxide, Sulphur dioxide and Nitrogen dioxide. Hence it has reduced pollution in our cities.

Q. Compare LPG and Wood as fuels?

Ans: LPG is obtained from petroleum refining whereas wood is obtained from forests.

LPG has more calorific value than wood.

LPG does not produce smoke like wood.

LPG has low ignition temperature.

Q. Give reasons:

A. Water is not used to control fires involving electrical equipment.

Ans: Because water is good conductor of electricity. Hence, water can give electric shock to firemen.

B. LPG is better domestic fuel than wood

LPG has more calorific value than wood. It does not produce smoke and it also has low ignition temperature.

C. Paper by itself catches fire easily where as paper wrapper around Aluminium pipe does not.

Ans: The aluminium is good conductor of heat. The heat from paper is passed to aluminium foil and paper does not reach its ignition temperature.

Q. Make the labelled diagram of candle flame and name different zones of flame.

Ans:

Make the labelled diagram of candle flame and name different zones of flame. Class 8th

The different zones of flame are:

Inner zone

This zone is least hot. It is black due to the presence of unburnt wax vapours.

Middle zone

This zone is moderately hot

Outer zone

This is the hottest zone. In this zone, complete combustion of fuel takes place.

Q. Name the unit in which calorific value of fuel is expressed?

Ans: The calorific value of a fuel is expressed in kilojoule per kg.

Q. Explain how CO2 is able to control fires?

Ans: The CO2 is lighter than O2. When sprayed over fire, It engulfs the flame and stops oxygen from reacting with fuel.

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

Ans: The green leaves contain large amount of water where as dry leaves have no water inside. Upon burning, water inside green leaves prevents them from reaching their ignition temperature. Hence, they are difficult to burn.

Q. Which zone of flame does goldsmith use to melt gold and silver and why?

Ans: The goldsmith uses outer zone of flame to melt gold and silver because it is the hottest zone of flame.

Q. In an experiment 4.5 kg off fuel was burnt. The heat produced was measured to be 1,80,000 KJ. Calculate the calorific value of fuel?

Ans:

Heat produced by 4.5 kg of fuel =180000 kJ.

Heat produced by 1 kg fuel i.e calorific value=

180000

4.5

=40000 kJ/kg.

Hence, the calorific value of the fuel is 40,000 kJ/kg.

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

Ans: Rusting cannot be called as combustion because combustion produces heat where as in rusting, no heat is produced.

Rusting is also very slow as compared to combustion.

Q. Abid and Ramesh were doing an experiment in which water was to be heated in a beaker. Abid kept the beaker near the wick in the yellow part of the candle flame. Ramesh kept the beaker in the outermost part of flame. Whose water will get heated in shorter time ?

Ans: The beaker of ramesh will get heated in shorter time. This is because outermost zone of flame is very hot than middle zone.

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