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Fire Control- Chemistry Guide for Class 8

Fire Control- Class 8 Science Guide

Information about Fire Control

Title

Fire Control

Class

Class 8

Subject

Class 8 Chemistry

Topics Covered

  • Fire Control and Safety
  • Incomplete Combustion
  • Flame


Fire Control 

We have now learnt that combustion takes place only if:
  1. there is a combustible material;
  2. there is a continuous supply of air;
  3. the temperature of combustible material is higher than its ignition temperature.

If any one of the above condition is not satisfied, combustion will not take place. This principle is used in 'fire fighting'.
In any kind of fire, it is some combustible material which catches fire. We must take precautions so that the relevant combustible substance does not get heated up to its ignition temperature. Places, like petrol stations, have highly combustible substances (like petrol, diesel, etc.) No one should be allowed to take any burning material within the premises of such places. Even a burning matchstick can ignite petrol vapours. The same precaution should be taken at LPG godowns, fire cracker factories, ammunition depots and even at our homes. For example, we need to take care while using a burning candle in the house during times of 'power failure'.
  • A fire, that has started, can be extinguished by either cutting off the supply of air, or by lowering the temperature of the combustible substance below its ignition temperature. For example, if the clothes of a person catch fire, the person should be immediately wrapped in a thick blanket.
  • This will cut off the supply of air and the fire will stop/die down.
  • Sand can be used to cut off the supply of air in case of fire produced by kerosene/petrol, etc.
  • Water can also be poured over the burning material to lower its temperature below its ignition temperature. However, water should not be used to extinguish fire caused by electric short circuits; this can cause dangerous 'electric shocks. 
  • A fire, caused by oil or gas, needs to be extinguished by using a carbon dioxide fire extinguisher.

What Happens if Incomplete Combustion?

During incomplete combustion:
  1. A part of the unburnt carbon passes into the atmosphere in the form of soot. This not only wastes the fuel but also pollutes the atmosphere.
  2. carbon monoxide is formed. This gas is highly poisonous and causes respiratory problems; it may also prove to be fatal.

Flame

Flame may be defined as the region over which gases burn. When you burn a piece of paper, or a wax candle, a flame is produced.
On observing the flame of a lighted candle carefully, we find that it has three zones.
  1. Innermost zone
  2. Middle zone or Luminous zone
  3. Outermost zone or Non-luminous zone

1. Innermost zone

This zone consists of unburnt wax vapour given off by the molten wax. It is the coldest part of the flame. 

2. Middle zone or Luminous zone

In this zone, partial combustion of wax vapours takes place with the liberation of a lot of energy. This energy partly decomposes the wax vapour into carbon particles. This zone of the flame is hotter than the dark inner zone; it is yellow in colour.

3. Outermost zone or Non-luminous zone

It is a zone of complete combustion of wax vapours and carbon particles. The air, from the sides of the flame, mixes with unburnt wax vapours and carbon particles (from the luminous zone) and burns them completely to form carbon dioxide gas and water vapour. It is the hottest part of the flame.

Let us perform an activity to show the presence of wax vapours in the innermost zone of candle flame.

Activity 1
  • Take a candle of medium size and fix it on a table.
  • Light the candle.
  • When the flame of the candle becomes steady, introduce a thin glass tube in the dark inner zone as shown. 
You will notice that the glass tube gets filled with greyish white vapours, which start coming out from the other end of the glass tube. Now, bring a burning matchstick near the mouth of the tube. You will observe that the vapours catch fire and burn producing a flame similar to that of the candle flame.
 
This shows that wax vapours are present in the innermost zone of candle flame.

Let us next preform an activity to show that the luminous zone of the candle flame contains unburnt particles of carbon.

Activity 2
  • Take a candle and fix it on a table.
  • Light the candle.
  • Now, introduce a clear glass slide into the luminous part of the flame, by holding it with a pair of tongs.
  • Hold the slide in the same position for about 40 seconds and then remove it. 
You will observe a circular greyish black ring formed on the glass slide in which there is no deposition in the middle of ring. The black deposition is due to the unburnt carbon particles in the luminous zone of the flame. The centre of the ring does not have any carbon particles because this part was over the dark inner zone which does not have unburnt carbon particles.

Let us perform an activity to show that the non-luminous (outer zone) is the hottest part of a candle flame.

Activity 6
  • Hold a thin and long copper wire across the candle flame (as shown) for about 30 seconds.
We will notice that the copper wire gets red hot in the non-luminous part of the flame, it just gets blackened in the luminous part of the flame. 

This observation shows that the outermost non-luminous zone of the flame is the hottest zone. 

Important Points

  • A soda-acid type fire extinguisher works in the following manner:
    It contains an acid in a glass bottle and solid sodium hydrogen carbonate outside it. On striking the fire extinguisher against a hard surface, the glass bottle breaks and the acid mixes with sodium hydrogen carbonate.

    The following reaction occurs:
    2NaHCO2 + H2SO4 ⟶ Na2SO4 + 2H2O + 2CO2 
    The CO2 gas, being heavier than oxygen, covers the burning objects like a blanket. Since the contact between the fuel and oxygen is cut off, the fire gets controlled.
    Also, CO2 when released from the cylinder, 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. It is, therefore, an excellent fire extinguisher.

  • If the fire is caused by an electric short circuit, it should not be extinguished by pouring water or carbon dioxide foam. It is because the electric current will flow through water, thereby, giving a severe electric shock which may prove fatal. In such case, supply of electric current should be switched off and fire brigade should be called immediately.

  • Goldsmiths, while shaping gold into ornaments, direct the non-luminous part (hottest zone) of the flame of a lamp on the gold with the help of a metallic blow pipe. The temperature of this part is around 1300°C which is sufficient to melt gold at specific points. This helps them to give proper shape to the gold ornaments. 
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