Contents
Chemistry Topics encompass a wide range of subjects, including organic chemistry, inorganic chemistry, and physical chemistry.
What are the Conditions Necessary for Combustion?
We will now learn the various conditions which are necessary for combustion (or burning) of a substance to take place. These can also be considered to be the conditions necessary to start a fire.
There are three conditions which are necessary for combustion to take place. These are :
- Presence of a combustible substance (A substance which can burn)
- Presence of a supporter of combustion (like air or oxygen)
- Heating the combustible substance to its ignition temperature
We will now discuss these three conditions required for combustion (or burning) of substances in detail.
1. Combustible Substance
The presence of a combustible substance is necessary for combustion to take place. So, when fire starts in a room, we remove all the combustible substances like wooden furniture, clothes, books and papers, etc., quickly from the room so that the fire may not spread due to the presence of a large number of combustible substances. A combustible substance is actually the food for fire’.
2. Supporter of Combustion
The most common supporter of combustion which we have around us is air. So, we can say that air is necessary for combustion. We can demonstrate that air is necessary for combustion by performing a simple activity as follows.
Activity 1
Light a candle with a burning matchstick and fix it on a table. We will see that this candle keeps on burning [see Figure (a)].This uncovered candle keeps burning because it is getting continuous supply of fresh air from the surroundings. Let us now cover the burning candle with an inverted gas jar. We will see that the candle stops burning after some time.
In other words, the candle gets extinguished [see Figure(b)]. The candle stops burning (or gets extinguished) because the supply of fresh air to the burning candle is cut off by the gas jar cover. Since no fresh air is available to the burning candle, it stops burning (or gets extinguished). This observation shows that air is necessary for combustion (or burning) to take place.
We will now give some examples from our everyday life which will tell us that a supporter of combustion like air is necessary for combustion to take place.
(i) If burning charcoal is covered with a vessel, it stops burning after some time, that is, the charcoal fire gets extinguished after some time. Actually, when we cover the burning charcoal with a vessel, the supply of supporter of combustion (air) to the burning charcoal is cut off and hence the charcoal fire stops.
(ii) When the clothes of a person catch fire, the person is covered with a blanket to extinguish the fire. This is because when the burning clothes of a person are covered with a blanket, the supply of air to the burning clothes is cut off due to which the clothes stop burning – the fire gets extinguished.
From all the above examples we find that when the supply of supporter of combustion (air) to a burning substance is cut off, then the process of burning (or combustion) also stops. Thus, a supporter of combustion (like air) is necessary for combustion to take place. If, however, there is no supporter of combustion (like air), then a combustible substance cannot burn even if it is heated to its ignition temperature.
3. Ignition Temperature
Before a combustible substance can catch fire and burn, it must be heated to a certain minimum temperature by supplying heat from outside. The lowest temperature at which a substance catches fire and starts burning, is called its ignition temperature.
It is necessary to heat a combustible substance to its ignition temperature so that it may undergo combustion (or burn). The ignition temperature of paper is 233°C. This means that a piece of paper has to be heated at least to a temperature of 233°C so that it may catch fire and start burning. A combustible substance cannot catch fire (or burn) as long as its temperature is lower than its ignition temperature.
We usually use a burning matchstick (or lighter) to make a substance burn. This burning matchstick supplies heat to raise the temperature of the substance to its ignition temperature and make it burn. This will become more clear from the following example :
A piece of paper does not catch fire (or does not burn) at the room temperature because the ignition temperature of paper is much higher than the room temperature. When we hold a burning matchstick near the piece of paper, it starts burning (see Figure).
This is because the heat produced by burning matchstick heats the piece of paper to its ignition temperature and makes it burn (or undergo combustion). The ignition temperatures of different substances are different. So, different substances catch fire and burn at different temperatures. Some substances have low ignition temperatures whereas other substances have comparatively high ignition temperatures.
(i) Some of the substances having low ignition temperatures are : Paper, Splinters of wood, Dry grass (Straw), White phosphorus, Cloth (Fabrics), Alcohol, Kerosene, Petrol, LPG, CNG and Biogas. The lower the ignition temperature, the more easily the substance will catch fire. The substances which have very low ignition temperatures and can easily catch fire with a flame are called inflammable substances.
In other words, a substance which is easily set on fire is called inflammable. Some of the examples of inflammable substances are : Petrol, Alcohol, Liquefied petroleum gas (LPG), Compressed natural gas (CNG) and Biogas. All these inflammable substances have very low ignition temperatures.
The fuels having very low ignition temperatures are very dangerous to use. For example, petrol has a much lower ignition temperature than that of kerosene. Due to its very low ignition temperature, a can full of petrol catches fire very easily on being lighted with a matchstick and burns explosively. This is why, petrol is not used in stoves. On the other hand, kerosene has a comparatively higher ignition temperature due to which it burns smoothly in a kerosene stove.
(ii) Some of the substances having high ignition temperatures are : Coal, Charcoal, Log of wood and Cow-dung cakes (Uple). The substances having high ignition temperatures catch fire with difficulty. The substances having high ignition temperatures burn only on strong heating. They cannot be burnt directly by a lighted matchstick.
We will now describe how a matchstick is lighted. A matchstick does not catch fire and burn on its own at room temperature because the ignition temperature of matchstick is much higher than the room temperature. A matchstick is lighted by rubbing it on the rough surface provided on the side of the matchbox (see Figure).
The heat produced by friction raises the temperature of the chemicals present on the matchstick head to their ignition temperature. Due to this, the chemicals present on the head of the matchstick catch fire and the matchstick starts burning (see Figure).
Thus, a matchstick starts burning on rubbing it on the side of the matchbox because the heat produced by friction heats the chemicals at the head of the matchstick to their ignition temperature and make it catch fire.
Kerosene oil and wood do not catch fire on their own at room temperature. This is because the ignition temperatures of kerosene oil and wood are higher than the room temperature. Now, if kerosene is heated a little (say, by a burning matchstick), it will catch fire easily.
This is because kerosene oil has a comparatively low ignition temperature which is reached even on little heating. But if wood is heated a little, it does not catch fire. This is because wood has a much higher ignition temperature which is not reached by the little heat being supplied to it by a matchstick. From this we conclude that the ignition temperature of kerosene oil is much lower than that of wood.
Since kerosene oil has a low ignition temperature and it can catch fire easily, we have to take special care in storing kerosene oil. Sometimes we see cooking oil in a frying pan catching fire when the frying pan is kept on the burning gas stove for a long time. This happens because the cooking oil gets heated to its ignition temperature when kept over a burning stove for a long time.
We know that a matchstick can light a tiny splinter of wood but not a big log of wood. This can be explained as follows: A splinter of wood has a low ignition temperature. Now, a burning matchstick can produce sufficient heat to reach the ignition temperature of the splinter of wood (which is low), therefore, a matchstick can light (or burn) a splinter of wood directly.
The ignition temperature of a log of wood is high which cannot be reached by the small heat produced by a burning matchstick. So, a matchstick cannot light (or burn) a log of wood directly. In order to burn a log of wood (as in a chulha), a small fire is first started by burning straw (or dry grass) with a matchstick, and then the log of wood is placed over this fire. The considerable heat of this fire then heats the log of wood to its ignition temperature due to which the log of wood starts burning.
Coal has a high ignition temperature, so a coal fire cannot be started by using a lighted matchstick directly. This is because the small heat produced by burning matchstick is not sufficient to heat the coal to its ignition temperature (which is high). A coal fire is started indirectly as follows:
A piece of cloth is dipped in kerosene oil and some pieces of wood are arranged over it. When the kerosene soaked piece of cloth is ignited by a lighted matchstick, it starts burning. The heat produced by the burning of kerosene soaked cloth makes the pieces of wood to burn. The coal pieces are then placed over the burning wood pieces. The large heat produced by the burning wood pieces heats the coal to its ignition temperature due to which the coal also starts burning. This starts the coal fire.
Sometimes we hear of ‘forest fires’ which occur on their own.
The forest fires occur during the hottest summer days. This happens as follows : During extreme heat of summer, sometimes the ignition temperature of dry grass in the forest is reached, which makes the dry grass catch fire. From the burning grass, the fire spreads to bushes and trees, and very soon the whole forest is on fire (see Figure).
It is very difficult to control such forest fires. We will now describe an activity which shows that it is essential for a substance to reach its ignition temperature to catch fire’ and ‘start burning’.
Activity 2
Make two paper cups by folding two round sheets of paper. Keep one paper cup empty but put about 50 mL of water in the other paper cup. Heat both the paper cups separately over a candle flame (see Figure). We will see that the empty paper cup catches fire easily and starts burning [see Figure (a)].
On the other hand, the paper cup containing water does not catch fire.The water in this paper cup becomes hot gradually [see Figure (b)]. If we continue heating this paper cup, we can even boil the water in it (without the paper cup catching fire). These observations can be explained as follows :
(i) When we heat the empty paper cup over the candle flame, then the ignition temperature of paper is reached quickly. Due to this, the empty paper cup catches fire quickly and starts burning.
(ii) When we heat the paper cup containing water, then the heat supplied to the paper cup is transferred to water inside it by conduction. Due to the continuous transfer of heat from paper cup to water, the paper cup does not get heated too much and its ignition temperature is not reached.
So, in the presence of water, the ignition temperature of paper cup is not reached, and hence the paper cup does not catch fire (or does not burn).
We know that it is difficult to burn a heap of green leaves but dry leaves catch fire easily. This can be explained as follows : The green leaves contain a lot of water. This water does not allow the green leaves to get heated to their ignition temperature easily and makes the burning of green leaves difficult. On the other hand, since dry leaves do not contain water, they get heated to their ignition temperature easily and hence catch fire easily.