What is combustion? You must have seen coal burning in the air which produces carbon dioxide and water. When we burn a piece of magnesium in the air, Magnesium oxide is formed along with the production of heat and light.

 

When we burn a candle in the air, CO2 gas along with heat and light is produced. All the examples we have seen earlier are chemical processes in which a new substance is formed with the evolution of heat. A chemical process in which a substance reacts with oxygen to give off heat is known as combustion.

 

The burning process in earlier shown activities is called combustion, where oxides are formed along with the evolution of heat and light. The substances that undergo combustion are said to be combustible substances. These are also known as fuel.

 

Examples of such substances are

 

• Wood
• Paper
• Kerosene oil
• Straw
• Charcoal
• Matchstick etc

 

Fuel Efficiency

 

Children it is important to note that the same amount of fuel, when burnt does not necessarily produce the same amount of heat. Hence to measure the efficiency of different fuels, scientists have coined the term calorific value. The amount of heat energy produced on complete combustion of 1 kg of a fuel is called its calorific value.

 

The calorific value of a fuel is expressed in a unit called kilojoule per kg (kJ/kg). Here you can see the Calorific values of some fuels! Burning of Fuels Leads to Harmful Products. The increasing fuel consumption has harmful effects on the environment.

 

Let us see some of those harmful effects :

 

1. Carbon fuels like wood, coal, petroleum release unburnt carbon particles. These fine particles are dangerous pollutants causing respiratory diseases, such as asthma.
2. Incomplete combustion of these fuels gives carbon monoxide gas. It is a very poisonous gas. It is dangerous to burn coal in a closed room. The carbon monoxide gas produced can kill persons sleeping in that room.
3. Combustion of most fuels releases carbon dioxide in the environment. Increased concentration of carbon dioxide in the air is believed to cause global warming.
4. Burning of coal and diesel releases sulphur dioxide gas. It is an extremely suffocating and corrosive gas. Moreover, petrol engines give off gaseous oxides of nitrogen. Oxides of sulphur and nitrogen dissolve in rain water and form acids. Such rain is called acid rain. It is very harmful for crops, buildings and soil.

 

However, some steps have been taking to reduce the dependence on these fuels. The use of diesel and petrol as fuels in automobiles is being replaced by CNG (Compressed Natural Gas) because CNG produces harmful products in very small amounts. CNG is a cleaner fuel.

 

Types of Combustion

 

Some substances burn rapidly and this type of combustion is called rapid combustion. Some substances suddenly burn into flames without any apparent cause and this type of combustion is known as spontaneous combustion. Some substances when ignited produce heat, light, sound, and large amounts of gases, and this type of combustion is known as an explosion.

 

We know that a chemical process in which a substance reacts with oxygen to give off heat is called combustion. Let's learn about the conditions necessary for combustion.

 

Conditions necessary for combustion

 

For combustion first thing we require is combustible substances i.e. the substances which can burn. Examples of such substances are:-

 

• Wood
• Paper
• Kerosene oil
• Straw
• Match stick

 

If there is no combustible substance, then no combustion can take place.

 

Now let�s do an activity

� Take three candles, wooden block supports, and three chimneys. Set them as shown here
� In the first case chimney will be placed on the wooden supports.
� In the second and third cases chimney will not be placed on wooden supports
� Now light all the candles
� Cover the third chimney with a wooden block.

What do you observe?

In the first case where air can enter the chimney from below, the flame continues to burn.
In the second case, air does not enter the chimney from below. The flame flickers and produces smoke. In the third case, the flame finally goes off because of the absence of air around.

Let�s do one more activity-

Take a piece of burning coal and cover it with a glass. What do you observe? You will observe that the coal stops burning due to the absence of oxygen in the glass. So we can conclude that air or oxygen is essential for burning.

 

Now let�s perform one more activity-

� Make two paper cups, by folding a sheet of paper. Pour about 50 ml of water into one of the cups.
� Heat both the cups separately with a candle
What do you observe?
� The empty cup burns immediately while the cup containing water doesn�t burn.
� In fact if we keep on supplying heat, the water in the cup will start boiling.
� The heat supplied to the paper cup is transferred to the water by conduction and when all the water evaporates the paper catches fire.

 

Why the empty paper cup catches fire first?

Every substance catches fire at some temperature. The lowest temperature at which a substance catches fire is called its ignition temperature. When the empty cup of paper attains its ignition temperature, it catches fire. The paper cup containing water does not attain its ignition temperature till the water finishes that so it catches fire late.

Similarly, the match stick doesn�t burn itself, but rubbing it on the side of the matchbox achieves the ignition temperature and catches fire. Kerosene oil and wood do not catch fire at room temperature but if they are heated a little, they catch fire.

If wood is heated it may not catch fire but if we pour little kerosene oil over it, it will catch fire fast. The reason being that the ignition temperature of kerosene is low and its burning will help the wood to attain its ignition temperature and thereby helping it to catch the fire.

 

The substances which have very low ignition temperature and can easily catch fire with a flame are called inflammable substances e.g. petrol, LPG, etc. So we can conclude that the attainment of ignition temperature is necessary for combustion.

 

So three essential conditions for combustion are:

� Presence of combustible substance
� Presence of oxygen or air
� Attainment of ignition temperature

So if we want to stop fire or combustion we just have to eliminate anyone of the conditions required for combustion.

 

How do We Control Fire?

 

You must have seen or heard of fire breaking out in homes, shops, and factories. Do you know, what are the techniques that are used to control the fire? You have learned that there are three essential requirements for producing fire That is 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. The most common fire extinguisher is water. But waterworks only when things like wood and paper are on fire.

 

If electrical equipment is on fire, water may conduct electricity and harm those trying to douse the fire. Water is also not suitable for fires involving oil and petrol and also since water is heavier than oil, it sinks below the oil, and oil keeps burning on top. For fires involving electrical equipment and inflammable materials like petrol, carbon dioxide (CO2) is the best extinguisher.

 

CO2, being heavier than oxygen, covers the fire like a blanket. Since the contact between the fuel and
oxygen is cut off, the fire is controlled. The added advantage of CO2 is that in most cases it does not harm the electrical equipment. This CO2 is stored at high pressure as a liquid in cylinders.

 

Similar to the one, how LPG is stored in cylinders. When released from the cylinder, CO2 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. That is why it is an excellent fire extinguisher.

 

Another way to get CO2 is to release a lot of dry powder of chemicals like sodium bicarbonate (baking soda) or potassium bicarbonate. Near the fire, these chemicals give off CO2.

 

Read More- What is Flame? How A Flame Is Formed? Zones Of Flames - Class 8