What is Combustion?

What is Combustion?

Combustion: It is a chemical process in which a substance reacts with oxygen to give off heat is called combustion.

In other words: Combustion is a chemical process in which a substance reacts with oxygen and generates heat in the process.

Any material or substance that undergoes combustion is known as a combustible substance. It is also called fuel.

Combustible substances are those substances that undergo combustion. It means that these substances give off heat and sometimes light (as a flame or glow) when they react with oxygen. Examples - Petrol, Diesel, etc. The fuel may be in a solid, liquid, or gas state. Sometimes, light is also given off during combustion, either as a flame or as a glow.

Inflammable Substances:

Those materials which have low ignition temperature and catch fire easily are termed inflammable substances. They burn with a flame. Example - Petrol, LPG, Alcohol etc.

Why do we say that food is fuel for our bodies?

When we eat food, it gets broken down into simpler substances that react with oxygen and generate energy (or heat). Hence, food is referred to as ‘fuel’ for our bodies.

Conditions necessary for the Combustion to take place are:

Combustion requires fuel.

Combustion only takes place when there is a fuel that produces heat and light when it catches fire. For example, petrol and diesel.

Combustion requires air.

We can prove it with the help of the following experiment: A candle keeps burning in the open air. However, when we cover it with a glass or a jar, it only burns until the oxygen inside it is consumed. Once the oxygen is exhausted, the flame of the candle flickers off.

Combustion requires heat. The substance must reach its ignition temperature to catch fire. Ignition Temperature: the minimum temperature at which any material catches fire is known as ignition temperature.


  • If the temperature of a combustible substance is lower than the ignition temperature, then the substance will not burn.
  • Cooking oil catches 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 catch fire. But if the wood is heated a little, it would still not catch fire.


Since ages, matchsticks are in use. Long ago, Egyptians used small pieces of pinewood dipped in Sulphur as matches. These days matchsticks are a lot safer. Modern matchsticks are made up of a mixture of antimony trisulphide and potassium chlorate with some glue and starch applied to the head of the match. The rubbing surface has powdered glass and some red phosphorous. On the striking match against a rough surface, red phosphorous gets converted into white phosphorous and it reacts with potassium chlorate to ignite antimony trisulphate and so the combustion takes place


  • Fuel
  • Air (With Oxygen in it).
  • Temperature above the Ignition temperature

Ignition temperature is the lowest temperature at which a substance catches fire or starts burning.

How Do We Control Fire?

How do We Control Fire?

We can control the fire by one or more of the following:

  • Removing the fuel
  • Cutting off the air supply (or oxygen supply)
  • Cutting off heat or lowering the temperature of the fuel
  1. Fire Brigade Stations: In case of fire, fire brigades will extinguish the fire by sprinkling the water on the affected areas. The water will bring down the temperature below its ignition temperature. As a result, fire will stop spreading.
  2. Fire Extinguisher: Water is the most common fire extinguisher. But, it works only on things like wood, paper, etc. However, in case the fire is caught on electrical things then, water being a good conductor of electricity will destroy that equipment. Even water is not good in case of fires due to oil, petrol, etc. In that case, Carbon dioxide is the best extinguisher. This cuts off the air supply and t brings down the temperature below the ignition temperature as a result fire gets extinguished.
  3. Use of Blankets: if a person catches the fire, then blankets can be used to extinguish the fire.
  4. Forest Fires: when the temperature rises too high then the regions having dry grasses will catch the fire. This fire spreads rapidly from grasses to trees and eventually entire forest is on fire. And it is difficult to manage such fires.

How do fire extinguishers work?

Fire extinguishers are devices used to put out fires. They either cut off the air supply to fire or cool off the fuel (below the ignition temperature) or both.

The three main types of fire extinguishers are:

1. ‘Dry Powder’ Fire Extinguisher: This type of fire extinguisher contains a mixture of baking soda (sodium bicarbonate) and sand. When you throw it over the fire, the baking soda decomposes by its heat to produce carbon dioxide. Since CO2 is heavier than air, it descends to envelop the burning flame and cuts off its contact with air (and the oxygen supply).

2NaHCO3  → Na2CO+ H2O + CO2

2. ‘Soda-acid’ Fire Extinguisher: This fire extinguisher is a metallic cylinder that contains the sodium bicarbonate (NaHCO3) solution. At the bottom of the cylinder, the concentrated sulphuric acid (H2SO4) is placed in a thin sealed glass tube. A fixed wire gauze surrounds this tube. Below the tube, a plunger is placed with its sharp end touching the thin glass tube. On the top of the cylinder, there is a nozzle that is sealed with wax. 

When the plunger hits against the floor, its sharp end breaks the thin glass tube and the acid inside it reacts with sodium bicarbonate to produce carbon dioxide. CO2 forces the wax seal open and rushes out of the nozzle to put out the fire in the direction where the nozzle is pointed.

3. ‘Foam-type’ Fire Extinguisher: Like the soda-acid fire extinguisher, it uses sodium bicarbonate solution. However, in this case, a substance called Saporin or Turkey Red Oil is added to the solution to produce foam along with the gas from the nozzle. Since this foam is lighter than oil, it floats on the surface of the oil and cuts off its air supply. Hence, it is very effective in putting out oil fires.

When should we use (or not use) water to extinguish the fire?

When wood, paper and clothes are on fire, we can use water to extinguish them. Water lowers the temperature of burning material below ignition temperature and thus, the fire stops burning. We should not use water when electrical equipment is on fire as water may conduct electricity and give a shock to people dousing the fire. Also, it should not be used when oil or petrol catches fire as water is lighter than oil and petrol and sinks down. Oil and petrol keep floating on the top and keep burning.

What should we do when electrical equipment or inflammable materials (like petrol) catch fire?

Carbon Dioxide is the best fire extinguisher in such cases. CO2 is heavier than oxygen and hence, covers the burning material like a blanket and cuts off its oxygen supply. Also, it does not harm the electrical equipment. CO2 can be stored as a liquid in cylinders at high pressure. When it is released, it immediately expands, cools down, and envelopes the fire - bringing down the temperature of the fuel. One can also pour dry chemicals like sodium bicarbonate (or baking soda) or potassium carbonate on the fire as they release CO2 near a fire.

Types of Combustion


1. Rapid Combustion
In this combustion, the substances burns rapidly and produces light and heat. Gas burns quickly producing heat and light in the process. E.g. LPG. 
Example: Bring a burning matchstick near a gas stove in the kitchen. Turn on   the knob of the gas stove. We find that the gas burns rapidly.

2. Spontaneous Combustion
In this type of combustion, substances burst out into flames suddenly without any known reason. Combustion in which a material bursts into flames suddenly without applying heat. E.g. Phosphorus which burns at room temperature. Spontaneous combustion of coal dust often causes accidental fires in coal mines. Heat from the sun or lighting may also cause spontaneous forest fires. Examples: Many disastrous fires in coal mines result due to this kind of combustion. The heat rays coming from the sun or a lightning strike might be responsible for this kind of combustion.

3. Explosion
In this type of combustion, all of a sudden reaction results into heat, light and sound. Moreover, large amount of gas gets released. When a material bursts suddenly to produce heat, light and sound on the application of heat or pressure, it is called an explosion. E.g. Crackers and fireworks which release a large amount of gas too. 
Example: When a fire cracker is ignited, a sudden reaction takes place with the evolution of heat, light and sound with the large amount of gas.





Flame is a hot glowing body of ignited gas that is produced when something is on fire. The substances which vaporize while burning, give flames. It is a place where the combustion of fuel takes place.
Kerosene oil and molten wax are substances that give a flame while burningThis gas is called flame.

Some materials burn with a flame and some do not. Here is a table based on a general observation:


Structure of a Flame

Structure of a Flame

A flame has three zones:

  1. Outermost zone is blue in colour and is the hottest part of the flame. This is also the zone where complete combustion takes place.
  2. Middle zone is yellow in colour and is moderately hot. In this zone, partial combustion takes place.
  3. Innermost zone is black in colour and least hot. Here, we can find the unburned wax vapours of a candle.

Note: Kerosene oil and molten wax vapourize during burning but charcoal does not produce flame or vapourize when it burns.

Why do goldsmiths blow at the outermost zone of a flame for melting gold and silver?

The outermost zone of a flame is its hottest part. Gold and silver have high melting points and hence, goldsmiths blow at the outermost zone of the flame to melt gold and silver quickly.

What is a Fuel?

What is a Fuel?


Fuels are substances that give us heat which we use for domestic and industrial purposes, such as wood, kerosene, and petrol. The substance that undergoes combustion is known as fuel. Examples of fuels are charcoal, petrol, kerosene, wood, etc.

What will an ideal fuel or good fuel look like?

Ideally, a good fuel is one which:

  • has proper ignition temperature (neither too high nor too low),
  • does not produce undesirable or poisonous substances and cause pollution,
  • does not leave behind much ash,
  • is cheap,
  • is readily available,
  • produces a large amount of heat or have high calorific value,
  • has a moderate rate of consumption
  • is easily controllable (can be started or stopped as needed),
  • is easy to handle and transport, and
  • has a low moisture content (so that it burns easily).

Note: Some fuels are cheaper than others.

Characteristics of Good Fuel:

  • It should be easily available and cheap.
  • It should generate a large amount of heat.
  • It should not leave any unwanted matter after combustion.

Ideal Fuel
The fuel which satisfies all the characteristics of good fuel is termed as an ideal fuel. But probably, there is no fuel considered as an ideal fuel.

Fuel Efficiency

Fuel Efficiency

Fuel efficiency of a fuel depends on its calorific value.

  • The quantity of heat generated on combustion of 1 kg of a fuel is called its calorific value.
  • Its unit is kilojoules per kg (KJ/kg).
Calorific Value of Common Fuels We Use

If we look at the table, we will see that hydrogen seems to be the most efficient fuel followed by LPG. CNG and Methane come next, closely followed by Kerosene, Diesel and Methane. Cow dung cake seems to be the least efficient fuel here.

Uses of Fuels

Some of the ways in which common fuels are used are:

Gasoline is used in cars, scooters and other vehicles we use every day.

Natural gas is used in heating systems, water heaters, dryers, and stovetops in our homes.

Oil and natural gas are used in making several things that we use every day. Hydrocarbons, for example, are used in making plastics, pharmaceuticals and several other items we use daily.

Coal is the primary fossil fuel used in many thermal power plants to produce electricity.

If we look at the table, we will see that hydrogen seems to be the most efficient fuel followed by LPG. CNG and Methane come next, closely followed by Kerosene, Diesel and Methane. Cow dung cake seems to be the least efficient fuel here.

Uses of Fuels

Some of the ways in which common fuels are used are:

Gasoline is used in cars, scooters and other vehicles we use every day.

Natural gas is used in heating systems, water heaters, dryers, and stovetops in our homes.

Oil and natural gas are used in making several things that we use every day. Hydrocarbons, for example, are used in making plastics, pharmaceuticals and several other items we use daily.

Coal is the primary fossil fuel used in many thermal power plants to produce electricity.

Types of Fuels

Fuels can mainly be divided into three groups:

Liquid Fuels: Petroleum (which is a fossil fuel), crude oil (from which we get petrol or gasoline), diesel, kerosene oil etc.

Solid Fuels: Firewood, charcoal, coal (fossil fuel which is mined as steam coke or soft coke), dung cakes, tallow (animal fat), straw and other agricultural wastes, paraffin wax, camphor etc.

Gaseous Fuels: Most commonly used gas is LPG (Liquid Petroleum Gas) which we use as cooking gas at home. Some of the other commonly used gaseous fuels are:

We get CNG (Compressed Natural Gas) from natural oil wells.

Natural gas (or methane) gets released from the putrefying organic matter.

Butane gas is obtained from natural gas.

In the villages, animal dung and farm waste are used to produce biogas. Biogas is also collected from sewage plants.

When hard coke is heated and converted into coke, coal gas is produced.

Water-gas are produced by passing steam over red-hot coke.

Acetylene used a gas produced by adding calcium carbide to water. Its smell is a bit unpleasant but the flame it produces is so hot that it is used for cutting metals and welding purposes.


The increasing fuel consumption has harmful effects on the environment.

  • Carbon fuels cause many respiratory diseases. It releases unburnt carbon particles.
  • The partial burning of some fuels releases carbon monoxide, which is a poisonous gas. And this gas can kill a person if left in a room filled with this gas.

GLOBAL WARMING - Combustion of most fuels increases the amount of carbon dioxide in the atmosphere that has led to increase in the average temperature on the earth.

ACID RAIN - Due to the burning of coal and diesel, Chemicals like Sulphur dioxide and nitrogen dioxide are released into the air. The pollutants react with the water vapor present in the air and form sulphuric and nitric acid. When it rains, these acids are also present. Such kind of rain is called Acid Rain. It is very harmful to crops, buildings and soil.

Prevention from Acid Rain: 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. Carbon fuels like wood, coal and petroleum release ash and fine unburnt carbon particles in the air which can cause respiratory diseases like asthma. These fine particles are referred to as Suspended Particulate Matter (SPM). Incomplete combustion of fuels (such as coal, gasoline and other fossil fuels)releases carbon monoxide gas which is very poisonous and can kill people sleeping in the room where coal is burning. CO combines with hemoglobin in our blood to form carboxyhemoglobin and renders it incapable of transporting oxygen.

These fuels also release unburnt hydrocarbons, many of which are carcinogenic (cause cancer) and pose serious health hazards. Carbon dioxide released by most fuels during combustion is causing an imbalance in the atmosphere. Deforestation is also leading to a situation where there are fewer trees to absorb the carbon dioxide. This is leading to global warming. The burning of coal and diesel releases Sulphur dioxide gas which is corrosive in nature and causes irritation in the nose, throat and airways. It also causes shortness of breath, wheezing, and a feeling of tightness around the chest. Petrol engines release gaseous oxides of nitrogen. These sulphur and nitrogen oxides dissolve in rainwater to form acids and cause acid rain.

Global warming is the rising temperature of the Earth's atmosphere which is causing the melting down of glaciers making the sea levels rise and causing floods in coastal areas. It is caused due to the elevated levels of carbon dioxide in the atmosphere as CO2 absorbs infrared radiation emitted by the Earth and radiates it back to the atmosphere and raises its temperature. This is also known as the ‘greenhouse effect.

Acid Rain refers to the rain of acids that is very harmful to crops, soil, animals, and buildings. These acids are formed when sulphur and nitrogen oxides dissolve in rainwater. Acid rains also damage trees at high elevations (such as red spruce that grows 2,000 ft above sea level), cause acidification of lake or stream water, and damage forest soils. Irreplaceable buildings, statues and sculptures which are part of a nation’s cultural heritage can get destroyed due to acid rain.

Why do we not burn wood anymore?

Wood is a low-cost fuel and is easily available but burning it releases a lot of smoke which causes respiratory problems. Moreover, cutting of trees for wood (as fuel) also leads to deforestation. Hence, people now prefer to use coal or LPG as fuel instead.

Why should we use CNG as automobile fuel?

CNG is a cleaner fuel and causes much less pollution than petrol and diesel. Hence, we should use CNG-powered vehicles now.

How to Conserve Fuels?

To conserve fuels, we should:

  • Collect all material required while cooking at one place before switching on the gas
  • check the pressure of tyres regularly 
  • choose walking overusing cars or motorbikes for short distances
  • use public transportation for traveling instead of private vehicles.

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