This topic gives an overview of;
We use different kinds of fuel for various purposes at home, in industry and for running automobiles.Some of the fuels are like cowdung, wood, coal, charcoal, petrol, diesel, compressed natural gas (CNG), etc.
You are familiar with the burning of a candle. Note the difference between the burning of a candle and the burning of a fuel like coal. May be you were able to guess right: candle burns with a flame whereas coal does not. Similarly, you will find many other materials burning without a flame. Let us study the chemical process of burning and the types of flame produced during this process.
Recall the activity of burning of magnesium ribbon. We learnt that magnesium burns to form magnesium oxide and produces heat and light .
We can perform a similar activity with a piece of charcoal. Hold the piece with a pair of tongs and bring it near the flame of a candle or a Bunsen burner. We find that charcoal burns in air. We know that coal, too, burns in air producing carbon dioxide, heat and light.
A chemical process in which a substance reacts with oxygen to give off heat is called combustion. The substance that undergoes combustion is said to be combustible. It is also called a fuel. The fuel may be solid, liquid or gas. Sometimes, light is also given off during combustion, either as a flame or as a glow.
In the reactions mentioned above magnesium and charcoal are combustible substances.
Let us consider an example. 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 as non-combustible in a table. Name some more substances which are combustible and add those to the Table.
Let us investigate conditions under which combustion takes place.
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. Observe what happens to the flame. Now remove the blocks and let the chimney rest on the table. Again observe the flame. Finally, put a glass plate over the chimney . Watch the flame again. We find that for combustion, air is necessary. The candle burns freely in case (a) when air can enter the chimney from below. In case (b), when air does not enter the chimney from below, the flame flickers and produces smoke. In case (c), the flame finally goes off because the air is not available.
In the sun, heat and light are produced by nuclear reactions.
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. Charcoal stop burning after sometime. You might have heard that when the clothes of a person catch fire, the person is covered with a blanket to extinguish fire. Guess the reason for it.
Recall some of your experiences burning a piece of paper. You have to use paper or kerosene oil to start fire in wood or coal.During extreme heat of summer, at some places dry grasses catch fire. From grasses, it spreads to trees, and very soon the whole forest is on fire. It is very difficult to control such fires.This is called Forest Fire.
The lowest temperature at which a substance catches fire is called its ignition temperature.
We find that a combustible substance cannot catch fire or burn as long as its temperature is lower than its ignition temperature. Have you ever seen cooking oil catching fire when a frying pan is kept for long on a burning stove? Kerosene oil and wood do not catch fire on their own at room temperature. But, if kerosene oil is heated a little, it will catches fire. But if wood is heated a little, it would still not catch fire. Ignition temperature of kerosene oil is lower than that of wood. We need to take special care in storing kerosene oil. The following activity shows that it is essential for a substance to reach ignition temperature to burn.
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. If we continue heating the cup, we can even boil water in the paper cup. The heat supplied to the paper cup is transferred to water by conduction. So, in the presence of water, the ignition temperature of paper is not reached. Hence, it does not burn.
The substances which have very low ignition temperature and can easily catch fire with a flame are called inflammable substances. Examples of inflammable substances are petrol, alcohol, Liquified Petroleum Gas (LPG), etc. Can you list some more inflammable substances?
You must have seen or heard of fire breaking out in homes, shops and factories. If you have seen such an accident, write a short description in your note book. Also, share the experience with your classmates.
When a fire brigade arrives, what does it do? It pours water on the fire. Water cools the combustible material so that its temperature is brought below its ignition temperature. This prevents the fire from spreading. Water vapours also surround the combustible material, helping in cutting off the supply of air. So, the fire is extinguished.
You have learnt that there are three essential requirements for producing fire. These are: fuel, air (to supply oxygen) and heat (to raise the temperature of the fuel beyond the ignition temperature). Fire can be controlled by removing one or more of these requirements. The job of a fire extinguisher is to cut off the supply of air, or to bring down the temperature of the fuel, or both. Notice that the fuel in most cases cannot be eliminated. If, for instance, a building catches fire, the whole building is the fuel.
Bring a burning matchstick or a gas lighter near a gas stove in the kitchen. Turn on the knob of the gas stove.
We find that the gas burns rapidly and produces heat and light. Such combustion is known as rapid combustion. There are substances like phosphorus which burn in air at room temperature.
The type of combustion in which a material suddenly bursts into flames, without the application of any apparent cause is called spontaneous combustion. Spontaneous combustion of coal dust has resulted in many disastrous fires in coal mines. Spontaneous forest fires are sometimes due to the heat of the sun or due to lightning strike. However, most forest fires are due to the carelessness of human beings. It is important to remember that the campfires must be completely extinguished before leaving a forest after a picnic, or a visit.
We generally have fireworks on festival days. When a cracker is ignited, a sudden reaction takes place with the evolution of heat, light and sound. A large amount of gas formed in the reaction is liberated. Such a reaction is called explosion. Explosion can also take place if pressure is applied on the cracker.
Cite this Simulator: