COMBUSTION AND FLAME

Combustion:

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.

For ex. magnesium and charcoal are combustible substances. Food is also a fuel for our body, as in our body food is broken down by reaction with oxygen and heat is produced.

Sun produces its own heat and light. Is it also some kind of combustion? No, In the sun, heat and light are produced by nuclear reactions. For Combustion, air is necessary. Without air, combustion cannot take place.

Ignition Temperature:

Different substances catch fire at different temperatures. The lowest temperature at which a substance catches fire is called its ignition temperature.  A combustible substance cannot catch fire or burn as long as its temperature is lower than its ignition temperature.

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.

History of matchstick:

The history of the matchstick is very old. More than five thousand years ago small pieces of pinewood dipped in sulphur were used as matches in ancient Egypt. The modern safety match was developed only about two hundred years ago.

A mixture of antimony trisulphide, potassium chlorate and white phosphorus with some glue and starch was applied on the head of a match made of suitable wood. When struck against a rough surface, white phosphorus got ignited due to the heat of friction. This started the combustion of the match. However, white phosphorus proved to be dangerous both for the workers involved in the manufacturing of matches and for the users.

These days the head of the safety match contains only antimony trisulphide and potassium chlorate. The rubbing surface has powdered glass and a little red phosphorus (which is much less dangerous). When the match is struck against the rubbing surface, some red phosphorus gets converted into white

phosphorus. This immediately reacts with potassium chlorate in the matchstick head to produce enough heat to ignite antimony trisulphide and start the combustion.

How to Control Fire:

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.

These are three essential requirements for producing fire :

1. Fuel.
2. Air (To supply oxygen).
3. and Heat (to raise the temperature of the fuel beyond the ignition temperature).

Fire Extinguisher:

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 water works 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.Water is heavier than oil. So, 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.

How do we get the supply of carbon dioxide? 

It can be stored at high pressure as a liquid 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.

Types of Combustion:

1. Rapid Combustion: The combustion in which large amount of heat is produced within a short period of time is called  Rapid combustion. External heat source is required to initiate it. For ex, LPG gas burns rapidly and produces heat and light.
2. Spontaneous combustion: The type of combustion in which a material suddenly bursts into flames, without the application of any apparent cause is called spontaneous combustion. For ex, 
• phosphorus which burn in air at room temperature.
• 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.
3. Explosion: 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.

Flame:

The substances which vapourise during burning, give flames. For example, kerosene oil and molten wax rise through the wick and are vapourised during burning and form flames. Charcoal, on the other hand, does not vapourise and so does not produce a flame.

Do you know why goldsmiths use the outermost zone of flame for melting gold and silver? 

Actually, there are different zones in a flame and the outermost zone of flame is hottest. There are three different zones of a flame -dark zone, luminous zone and non-luminous zone.

When a clean glass plate is introduced into luminous zone of a candle flame for some time, we see a circular blackish ring formed on glass plate. It indicates the deposition of unburnt carbon particles present in the luminous zone of the flame.

Different zone of Flame

Fuel and Fuel Efficiency:

Wood, charcoal, petrol, kerosene, etc. are some example of fuels. A ideal fuel is one which 

• is readily available. 
• is cheap. 
• burns easily in air at a moderate rate. 
• produces a large amount of heat. 
• does not leave behind any undersirable substances.

However, There is probably no fuel that could be considered as an ideal fuel.

Fuel Efficiency: 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). Calorific values of some fuels are given below:

The increasing fuel consumption has harmful effects on the environment.

1. Carbon fuels like wood, coal, petroleum release unburnt carbon particles. These fine particles are dangerous pollutants causing respiratory disease, 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. 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.
4. Combustion of most fuels releases carbon dioxide in the environment. Increased concentration of carbon dioxide in the air is believed to cause global warming.

Global warming is the rise in temperature of the atmosphere of the earth. This results, among other things, in the melting of polar glaciers, which leads to a rise in the sea level, causing floods in the coastal areas. Low lying coastal areas may even be permanently submerged under water.

The use of diesel and petrol as fuels in automobiles is being replaced by CNG (Compressed Natural Gas), because CNG produces the harmful products in very small amounts. CNG is a cleaner fuel.

To read previous year CTET Question / Sample question, click here.