Combustion and flame Worksheet-8

Combustion and flame Worksheet-8

1. (a) Name the type of nuclear reaction taking place in atmosphere of Sun.

(b) Define ignition temperature.

(c) Why water should not be used to put out fire caused by burning of petrol?

2. What are characteristics of an ideal fuel. What is C.N.G.? Mention its one use.

3. Name three types of combustion. Give one example of each.  

4. What is combustion? What are the conditions required for combustion to take place?

5. (a) Which are the hottest zone and the coolest zone of a flame?

(b) Why does the middle zone of the candle flame glow with yellow colour?

Answer:

1. (a) Nuclear fusion takes place in the sun. The lighter elements combine to form heavier elements giving energy in nuclear fusion. In the core of Sun Hydrogen combines to form helium giving enough energy to sustain life on earth.

(b) The lowest temperature at which a substance catches fire is called ignition temperature.

(c) Water should never be thrown on a liquid fire

(i) Water does not mix with the oil, water being heavier than oil goes beneath them and substance continues burning.

(ii) Water boils and become steam very quickly. Since water is at the bottom of the oil, as it evaporates it sprays hot, burning oil in every direction. This then spreads the fire.

(iii) Liquid fires can be stopped by using Fire extinguisher.

2. A fuel should be-

It should have a high calorific value.

Its ignition temperature should be low but well above the room temperature. If the ignition temperature is too low, the fuel will catch fire very easily (which could be dangerous) and if it is very high, the fuel has to be heated for a long time before it can catch fire.

It should have a moderate rate of combustion and should release heat in a controlled manner.

It should be fairly cheap and easily available. A fuel may have a very high calorific value but if it is expensive and not easily available, it cannot be used on a day-to­day basis.

It should be safe to handle, store, and transport.

It should not cause pollution on burning.

C.N.G. is compressed natural gas. It is used as fuel in automobiles. It does not cause pollution.

3. There are three types of combustion depending upon the time span in which the substance burns releasing heat, light or produces sound.

Rapid combustion

In this type of combustion, large amount of heat and light are released in a very short span of time.

Example: Combustion of LPG, which produces heat and light instantly

Explosion 

Combustion in which a sudden reaction takes place with the evolution of heat, light, sound and large amount of gas is known as explosion e.g.  Ignition of fire crackers

Spontaneous combustion 

The type of combustion in which a material suddenly bursts into flames without application of any apparent heat is called spontaneous combustion. e.g. spontaneous combustion of coal dust, White phosphorous

4. A chemical process in which a substance reacts with oxygen producing heat is called combustion.

Conditions Necessary for Combustion

Combustible substance: Fuel

Air (to supply oxygen)

Heat (to raise the temperature beyond the ignition temperature)

A substance will not burn without one or more of these conditions.

5. A candle flame can be divided into three zones, depending on the amount of oxygen available.

The outer zone (blue) is the hottest part of the flame. In this zone, the wax vapours have enough oxygen to burn completely (producing carbon dioxide and water).This zone emits very little light.

The middle zone (yellow) is less hot than the outer zone. Here, incomplete combustion of wax vapours (due to low oxygen) produces carbon particles (which glow, giving the zone its yellow colour) and carbon monoxide. This zone emits the most light.

The inner zone (black) is the coolest part of the flame. In this zone, the wax vapours remain unburnt as no oxygen is available. This zone is completely dark and emits no light.