We use various types of substances as sources of energy for different purposes in our homes, factories, and vehicles. These substances are commonly known as fuels.

Examples of fuels used in homes include wood, cow dung cakes, coal, kerosene, and LPG (Liquified Petroleum Gas). Industries and trade might use coal, diesel, natural gas, etc. Automobiles typically use petrol, diesel, or Compressed Natural Gas (CNG).

Have you observed how a candle burns compared to how coal burns? A candle typically burns with a visible flame, while coal burns without a flame, producing a glow instead. Many other substances also burn without a flame. This chapter explores the chemical process of burning and the characteristics of flames produced during this process.

It is worth noting that even the food we eat is considered a 'fuel' for our body. Our body breaks down food through reactions involving oxygen, releasing energy in the form of heat, which is essential for our bodily functions.

What Is Combustion?

Combustion is defined as a chemical process in which a substance reacts with oxygen, usually from the air, to produce heat. Often, light is also produced during combustion, either as a flame or simply as a glow.

The substance that undergoes this process of burning is called a combustible substance. Combustible substances are also referred to as fuels. Fuels can exist in solid, liquid, or gaseous states.

For example, when magnesium ribbon burns in air, it reacts with oxygen to form magnesium oxide, releasing heat and light. Similarly, charcoal burns in air, reacting with oxygen to produce carbon dioxide, heat, and light.

Examples of combustible substances include wood, paper, straw, charcoal, kerosene oil, and matchsticks. Substances like iron nails, stone pieces, and glass are non-combustible as they do not burn in air.

For combustion to occur, three essential conditions or requirements must be met:

1. Fuel: The combustible substance itself.
2. Oxygen: Usually supplied by air, it is necessary for the chemical reaction.
3. Heat: To raise the temperature of the fuel to its ignition temperature.

Experiments demonstrate the necessity of air (oxygen) for burning. If the supply of air is cut off from a burning object, the flame will eventually go off.

Have you noticed that a matchstick does not spontaneously catch fire at room temperature? But it burns when rubbed against the side of a matchbox. A piece of paper burns easily when a lit matchstick is brought near it, but a piece of wood might require paper or kerosene to start burning.

These observations indicate that different substances require different temperatures to start burning. The lowest temperature at which a substance catches fire and begins to burn is called its ignition temperature.

A matchstick head ignites when rubbed because the friction generates enough heat to reach the ignition temperature of some of the chemicals in the matchstick head, initiating combustion.

Consider heating water in a paper cup over a candle flame. The paper cup containing water does not catch fire easily, even though paper is combustible. This happens because the heat supplied by the candle is transferred to the water by conduction. As long as the water absorbs the heat, the temperature of the paper does not reach its ignition temperature, thus preventing it from burning. If the heating continues, the water in the cup might even boil.

Substances that have a very low ignition temperature and can easily catch fire with a flame are called inflammable substances. Examples include petrol, alcohol, and LPG. Special care is needed when storing and handling these substances.

How Do We Control Fire?

Fires can cause significant damage to homes, businesses, and factories. It is important to know how fires can be controlled.

Since fire requires fuel, oxygen (from air), and heat (to reach ignition temperature), a fire can be controlled by removing one or more of these essential requirements.

A device used to extinguish fire is called a fire extinguisher. The main function of a fire extinguisher is typically to cut off the supply of air to the burning material or to lower the temperature of the fuel below its ignition point, or both.

In most fire incidents, the fuel itself (e.g., a building structure) cannot be removed.

The most commonly used fire extinguisher is water. Water cools the combustible material, reducing its temperature below its ignition point, which stops the fire from spreading. Water also turns into steam, which surrounds the burning material, helping to block the supply of air (oxygen).

However, water is not suitable for all types of fires:

• For fires involving electrical equipment, water can conduct electricity, posing a serious risk of electric shock to those fighting the fire.
• For fires involving oil or petrol, water is heavier than these liquids. When sprayed on oil or petrol fires, water sinks below the burning liquid, and the oil/petrol continues to burn on the surface.

For electrical fires and fires involving inflammable liquids like petrol, carbon dioxide (CO$_2$) is the most effective fire extinguisher. CO$_2$ is heavier than oxygen, so it forms a blanket around the burning material, cutting off the supply of air. Additionally, CO$_2$ released from a cylinder expands and cools down, helping to lower the temperature of the fuel. A significant advantage of using CO$_2$ is that it generally does not harm electrical equipment.

Carbon dioxide can be stored under high pressure as a liquid in cylinders. It can also be generated at the site of the fire by spraying dry powders of chemicals like sodium bicarbonate (baking soda) or potassium bicarbonate, which release CO$_2$ when heated.

Types Of Combustion

Combustion can occur in different ways, leading to different classifications:

1. Rapid Combustion: This type of combustion occurs quickly and produces heat and light readily. When you turn on a gas stove and bring a lighter near it, the gas burns rapidly, demonstrating rapid combustion.
2. Spontaneous Combustion: This is a phenomenon where a material suddenly ignites and bursts into flames without any apparent external trigger or application of heat. Substances like white phosphorus undergo spontaneous combustion at room temperature when exposed to air. Spontaneous fires can occur in coal mines due to the self-ignition of coal dust. Forest fires can sometimes start spontaneously due to extreme heat from the sun or lightning strikes, though often they are caused by human carelessness.
3. Explosion: This occurs when a substance undergoes a very rapid reaction that produces heat, light, and a large amount of gas, often accompanied by a loud sound. Fireworks are a common example; when ignited, they explode. Explosions can also happen if pressure is applied to certain explosive materials.

Flame

Some substances burn with a visible flame, while others burn without one, producing only heat and light as a glow. Examples of substances that produce a flame on burning include LPG (gas stoves) and candle wax. Substances like coal and charcoal typically burn without a flame.

The presence or absence of a flame depends on whether the substance vapourises during burning. Substances that vapourise during combustion produce flames. For instance, kerosene oil and the molten wax in a candle wick vapourise as they burn, forming a flame. Charcoal, however, does not vapourise and therefore burns with a glow.

Structure Of A Flame

A flame is not uniformly structured; it consists of different zones, each with varying characteristics like temperature and the extent of combustion.

Observing a steady candle flame reveals its distinct regions:

• Innermost Zone (Dark Zone): This is the region closest to the wick. It consists of unburnt wax vapours. It is the least hot part of the flame and appears dark.
• Middle Zone (Luminous Zone): This is the central, bright, yellow part of the flame. In this zone, there is insufficient oxygen for complete combustion, leading to partial combustion of wax vapours. This incomplete burning produces fine particles of unburnt carbon (soot), which glow and make the flame visible and luminous (bright). This zone is moderately hot. Holding a glass plate in this zone will collect soot particles, forming a black ring.
• Outermost Zone (Non-luminous Zone): This is the thin, outer layer of the flame, often appearing blue. In this zone, there is plenty of oxygen available from the surrounding air, leading to complete combustion of the fuel vapours. This complete burning releases the maximum amount of heat, making this the hottest part of the flame. It is the least visible (non-luminous) because less soot is produced here.

Goldsmiths often use a metallic blow-pipe to direct the outermost zone of a flame onto gold and silver to melt them because this zone is the hottest, providing the necessary temperature.

What Is A Fuel?

As previously mentioned, substances that provide heat energy for various applications are called fuels. Common examples include wood, charcoal, petrol, kerosene, coal, natural gas, and LPG.

An ideal fuel would possess several desirable characteristics:

• It should be easily available and inexpensive.
• It should burn readily in air at a moderate rate.
• It should produce a large amount of heat.
• It should not leave behind harmful or undesirable residues after burning.

In reality, a fuel that perfectly fits all these criteria likely doesn't exist. Therefore, we select a fuel that best meets most requirements for a specific use.

Fuels also differ in cost.

Fuels can be classified based on their physical state:

Solid Fuels	Liquid Fuels	Gaseous Fuels
Wood	Kerosene oil	Natural gas
Coal	Petrol	LPG
Charcoal	Diesel	CNG
Cow dung cake		Biogas

Fuel Efficiency

Different fuels release different amounts of heat energy upon complete burning. The efficiency of a fuel is measured by its calorific value.

The calorific value is defined as the amount of heat energy produced by the complete combustion of 1 kilogram (or 1 gram for smaller quantities) of a fuel.

The unit for expressing calorific value is kilojoule per kilogram (kJ/kg).

A higher calorific value means that a smaller mass of the fuel is needed to produce a larger amount of heat energy.

Examples of calorific values for some fuels:

Fuel	Calorific Value (kJ/kg)
Cow dung cake	$6000 - 8000$
Wood	$17000 - 22000$
Coal	$25000 - 33000$
Petrol	$45000$
Kerosene	$45000$
Diesel	$45000$
Methane	$50000$
CNG	$50000$
LPG	$55000$
Biogas	$35000 - 40000$
Hydrogen	$150000$

Burning Of Fuels Leads To Harmful Products

While providing energy, the increasing use of fuels has several negative consequences for the environment and human health:

1. Unburnt Carbon Particles: Fuels containing carbon (like wood, coal, petrol, diesel) often release fine, unburnt carbon particles, especially during incomplete combustion. These are dangerous air pollutants that can cause respiratory diseases such as asthma.
2. Carbon Monoxide Gas: Incomplete combustion, which occurs when there is insufficient oxygen, produces carbon monoxide (CO). This is a highly poisonous gas. Burning fuels like coal in a closed room is very dangerous because the accumulated carbon monoxide can cause death, as it interferes with the body's ability to transport oxygen.
3. Carbon Dioxide Gas: The combustion of most fuels releases carbon dioxide into the atmosphere. While carbon dioxide is a natural component of air, its concentration has increased significantly due to increased fuel burning. This rise in atmospheric carbon dioxide is strongly linked to global warming.

   Global warming refers to the increase in the average temperature of the Earth's atmosphere. This phenomenon has serious consequences, including the melting of polar ice and glaciers, which leads to a rise in sea levels and increased risk of flooding in coastal areas. Low-lying coastal regions could potentially be permanently submerged.

4. Oxides of Sulphur and Nitrogen: Burning coal and diesel releases sulphur dioxide ($\text{SO}_2$), a suffocating and corrosive gas. Petrol engines emit gaseous oxides of nitrogen ($\text{NO}_x$). These gases react with water vapour in the atmosphere to form acids (sulphuric acid and nitric acid). When these acids fall to the Earth with rain, it is called acid rain.

   Acid rain is harmful to crops, damages buildings (particularly those made of marble), and affects the quality of soil and water bodies.

Due to these harmful effects, there is a growing shift towards using cleaner fuels. For instance, in automobiles, petrol and diesel are increasingly being replaced by CNG (Compressed Natural Gas) because CNG produces significantly fewer harmful products during combustion, making it a much cleaner fuel.

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