Combustion and Flame

Activity Solution

Activity 6.1

Collect some materials like straw, matchsticks, kerosene oil, paper, iron nails, stone pieces, glass etc. Under the supervision of your teacher try to burn each of these materials one by one. If combustion takes place mark the material combustible, otherwise mark it non-combustible (Table 6.1)

After activity 6.1

Can you name some more substances which are combustible? You can add those to Table 6.1

Table 6.1: Combsutible and Non-combustible substances

Answer

Activity 6.2

(Caution: Be careful while handling burning candle).

Fix a lighted candle on a table. Put a glass chimney over the candle and rest it on a few wooden blocks in such a way that air can enter the chimney [Fig. 6.2 (a)].

Fig. 6.2: Experiment to show that air is essential for burning

Now remove the blocks and let the chimney rest on the table [Fig. 6.2(b)]. Again observe the flame. Finally, put a glass plate over the chimney [Fig. 6.2 (c)]. Watch the flame again.

What happens in the three cases? Does the flame flicker off? Does it burn unaffected?

Can you infer anything all about the role played by air in the process of burning?

Answer

(a) In first case, candle burns without any problem.

(i) Flame does not flicker off.

(ii) Candle burns unaffected.

Candles burns continuously because air flows steadily in the chimney without any interruption from the space available at the bottom because of wooden blocks.

(b) In second case, candle burns for few seconds only.

(i) Flame of candle flicker off.

(ii) Candle does not burn unaffected.

Candles flicker off and produce smoke because air is not coming from the bottom of chimney.

(c) In third case,

(i) The flame goes off due to unavailability of air (oxygen) and smoke is produced.

The role of air in the process of burning

Oxygen present in air supports combustion. Without oxygen burning is not possible.

Activity 6.3

Place a piece of burning wood or charcoal on an iron plate or Tawa. Cover it with a glass jar or a tumbler, or a transparent plastic jar. Observe what happens. Does charcoal stop burning after sometime? Can you think of the reason why it stops burning?

Answer

Yes after sometime charcoal stops burning. As burning charcoal or wood is covered with a glass jar or a tumbler, charcoal burns till oxygen present in the jar. In the course of burning carbon dioxide is produced by charcoal also. After sometime oxygen present in jar get exhausted due to burning and simultaneously charcoal is covered with carbon dioxide, thus, charcoal stop burning.

Activity 6.4

(Caution: Be careful while handling burning candle. )

Make two paper cups by folding a sheet of paper. Pour about 50 mL of water in one of the cups. Heat both the cups separately with a candle (Fig. 6.5). What do you observe?

What happens to the empty paper cup? What happens to the paper cup with water? Does water in this cup become hot?

If we continue heating the cup, we can even boil water in the paper cup.

Can you think of an explanation for this phenomenon.

Answer

Paper cup which contains water do not burns and paper cup which does not contain water starts burning quickly.

Paper cup which contains water do not burn till water is vapourised. This happens because heat introduced to paper cup containing water, goes to water. Additionally water keeps the ignition temperature of the paper cup below limit. This the cause that paper cup containing water does not burn.

On the other hand paper cup having no water, reached its ignition temperature required to burn paper quickly and it starts burning.

Activity 6.5

Light a candle (Caution: Be careful). Hold a glass tube with a pair of tongs and introduce its one end in the dark zone of a non-flickering candle flame (Fig. 6.10). Bring a lighted matchstick near the other end of the glass tube. Do you see a flame? If so, what is it that produces a flame? Notice that the wax near the heated wick melts quickly.

Answer

Yes a flame is seen at the other end of glass tube. And this flame keeps on burning.

The vapour of wax starts coming out through glass tube and when a lighted matchstick is brought near the other end of glass tube, the vapour of wax starts burning.

The wax near the heated wick melts quickly because of heat transfer form glass rod near the candle.

Important Keywords and their explanation related to the chapter of Combustion and Flame

(1) Acid rain

The burning of fossil fuels releases carbon dioxide, sulphur dioxide and oxide of nitrogen in air. Carbon dioxide produces carbonic acid, sulphur dioxide produces sulphuric acid and nitrogen oxide produces nitric acid when mix with water. In the course of raining these gases are mixed with water and in the form of acid come down to the earth. This phenomenon is called acid rain.

Acid rain is very harmful for buildings, water, soil and crops.

(2) Calorific Value

The efficiency of a fuel is measured in calorific value. The unit of calorific value is kilo jule per kg (kJ/kg).

(3) Combustion

Combustion is a chemical process in which a substance reacts with oxygen to give off heat.

(4) Deforestation

Removal of forest is known as deforestation. Deforestation may be man made or due to natural calamities.

(5) Explosion

A sudden reaction with the evolution of heat, light and sound is called explosion. In explosion a large amount of heat is liberated.

(6) Flame

Visible gaseous part of fire coming out from combustible is called Flame.

(7) Fire extinguisher

Substance which put the fire off is called fire extinguisher. Water is most popular and cheaper fire extinguisher. CO₂ is the most efficient fire extinguisher.

(8) Fuel

Combustible substances are known as fuel. Fuel may be solid, liquid or gas. But all combustible substances are not considered as fuel rather substances which are used as the source of heat energy for domestic and industrial purposes are called fuels. For example wood, charcoal, petrol, kerosene, etc.

(9) Fuel Efficiency

Fuel that produces more heat in the less amount of fuel is called more efficient fuel. Fuel efficiency is measured in the term of Calorific Value.

(10) Global warming

The rise in temperature of the atmosphere of the earth is called global warming.

(11) Ideal fuel

A good fuel is one which is readily available, cheap, burns easily in air at a moderate rate, produces a large amount of heat and does not leave behind any undesirable substance.

Thus, no fuel is considered as an ideal fuel. Howeve fuel which fulfils most of the requirement is considered as an ideal fuel.

(12) Ingnition temperature

Minimum temperature at which a substance catches fire is called ignition temperature of that substance.

(13) Inflammable substances

Substances which have very low ignition temperature and can easily catch fire with a flame are called inflammable substance.

8-science-home

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