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Combustion is a chemical process where a substance reacts with oxygen readily to give out heat and light. It does not always result in the formation of fire. When heat is given out, it can be used as an energy source known as fuel. The objects which easily catch fire are called combustible materials. These combustible materials act as combustion fuels. Some examples of combustible materials are petroleum, diesel, natural gas, wood, dried leaves, natural fibres, etc.
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Key Terms: Combustion, Fuel, Ignition temperature, Fire, Flames, Inflammable substances, Water, Calorific value, Carbon, Temperature, Pressure
What is Ignition Temperature?
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Combustible substances require a particular temperature to react with oxygen. The minimum or the lowest temperature at which a substance catches fire is called its ignition temperature. Every substance will have a different ignition temperature. For eg, a matchstick when kept by itself will not burn but when it is rubbed against a combustible coated surface, the heat produced by the friction causes the matchstick to catch fire.
Match stick ignition
While comparing kerosene oil and wood with fire, one will realise that when exposed to heat, kerosene oil catches fire faster than wood. Thus the ignition temperature of kerosine oil is lower than wood. When exposing a wet paper to fire along with a dry paper, the dry paper will catch fire faster than the wet paper because the moisture in the wet paper has altered the ignition temperature of the wet paper.
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| Exothermic Reaction | Oxidizing Reaction | |
Types of Combustion
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Based on the type of reactions that substances show when they come in contact with fire, they are classified into the following combustion –
- Rapid Combustion: When a substance exposed to fire, immediately catches fire, undergoes rapid combustion. Eg, Liquid petroleum gas.
- Spontaneous Combustion: When a substance does not require any external heat source for ignition but catches fire as soon as it comes in contact with atmospheric oxygen undergoes spontaneous combustion. Eg, Sodium metal, Phosphorous.
- Explosion Combustion: Substances that catch fire with a release of heat, gas, light, and sound undergo explosion combustion. Eg, Crackers
Combustion Types
- Complete Combustion: A substance undergoes complete combustion when the oxygen in air is adequate to combust all of the fuel present. The temperature is high enough to consume all of the substance.
- Incomplete Combustion: A substance undergoes incomplete combustion when there is inadequate oxygen present in the air and the temperature is also not enough to consume all of the substance. Thus parts of the substance remains unburnt.
Combustion Conditions
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Inorder for combustion reaction to take place, the following conditions should be met.
- Presence of Oxygen.
- Sources of heat to keep the temperature of the substance above its ignition temperature.
- Presence of fuels.
- Presence of combustible material.
- Absence of moisture, water, and other non-inflammable substances to continue burning of the substance.
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| Critical Temperature | Rate of Reaction | |
Flames
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While observing the flame of a candle and kitchen stove, we can see the difference in colour and shape of the flame. The colours of the flame and the presence or absence of flames are due to the chemical reactions undergoing on that substance during combustion. While burning some substances give off flames while some do not. The vapours from the combustible substance are responsible for flames from the substance. Vaporization occurs due to evaporation.
Parts of a Flame
The part from where the flame originates contains the unburnt wax and is called the dark zone. The innermost part of the flame has the least heat and is known as the luminous part, as it gives off maximum light. The centre part of the flame is moderately hot while the outermost part of the flame, the border of the flame forms the hottest part of the flame. The outermost part of the flame is known as the non-luminous part.
Fuel
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A fuel is known as the energy source for combustion. An ideal fuel should be-
- Easily available
- Non-expensive
- Produces a large amount of heat i.e. energy at normal burning efficiently
- Does not require a special atmosphere to burn
- Easily burns in moderate oxygen
- Does not leave behind undesirable residue (wastes) after burning
- Easy to transport
- Has high calorific value
The calorific value (efficiency) of the fuel is decided on the amount of heat energy released during complete combustion of 1 kg fuel. The higher the calorific value, the more is the efficiency of the fuel. It is measured in kJ/kg.
Also Read:Types of Fuel
Things to Remember
- Inflammable substances have very low ignition temperature, meaning they can quickly catch fire.
- Flames and fire are different, as fire burns on the substance itself, whereas flames appear due to the vaporization of the substance.
- Wood, charcoal does not vaporize while burning thus they do not give flames.
- The non-luminous zone has the highest temperature of the flame.
- Till date, there is no ideal fuel found.
Sample Questions
Ques. Explain Smoke, soot [2 marks]
Ans. When a substance burns, we can observe a dark gaseous release just above the flame. This gas contains the unburnt particles of the combustible substance. This is known as smoke. It is one of the major pollutants.
When you heat a vessel, you will observe a black coloured powders substance formed at its base on the side exposed to heat, this is the carbon residue which is the soot.
Ques. Distinguish between LPG (Liquid Petroleum Gas) and wood [3 marks]
Ans.
| LPG | Wood |
|---|---|
| It does not leave behind residue after burning | It leaves behind residue |
| It is a gaseous fuel | It is a solid fuel |
| Easy to store and transport | Cannot be transported as easily as LPG |
| Low ignition temperature | High ignition temperature |
| Does not release smoke while burning | Releases smoke while burning |
| Burns very easily | Takes time to catch fire |
Ques. What is a fire extinguisher, and how does it work? [2 marks]
Ans. A fire extinguisher is a device which is used toput out fire. It cuts off the oxygen supply from the air to the burning substance. Without further oxygen supply, the substance stops burning. Some fire extinguishers like water are used to lower the temperature of the substance thus increasing its ignition temperature. The aim of the fire extinguisher is to control fire.
Ques. How does the combustion of fuels contribute to pollution? [3 marks]
Ans. The fuels that we use in our day-to-day life are mostly natural gas, or fossil fuels which are exhaustible fuels. While burning these fuels, they give off unburnt carbon particles into the environment which acts as dangerous pollutant. Incomplete combustion which happens in oxygen deficit areas contributes to the production of carbon monoxide which is a poisonous gas. While burning these fuels release carbon dioxide in the atmosphere which increases global warming. The fuel residue gets mixed with water and does not decompose instead creates water pollution. The smoke given out cause air pollution.
Ques. What are the conditions required for combustion? [2 marks]
Ans.
- Presence of Oxygen
- Heat sources to keep the temperature of the substance above its ignition temperature.
- Presence of fuels
- Presence of combustible material
- Absence of moisture, water, other non-inflammable substances to continue burning of the substance.
Ques. Why has CNG helped in reducing pollution? [2 marks]
Ans. The number of vehicles are rapidly increasing and so is the amount of fuel usage. Thus to avoid the increase in pollution caused by the use of fuels, CNG – Compressed Natural Gas is used as a reliable alternative. It gives off less amounts of carbon dioxide gas, sulphur dioxide, carbon monoxide, and nitric dioxide than other fuels like petrol, diesel. It increases the life of the engine. It does not leave behind harmful residue after its use. It is known as a clean fuel. Many vehicles have now started using CNG as it is fuel as it is both cost-effective and less pollutant.
Ques. Why should you not use water to distinguish electrical equipment on fire? [2 marks]
Ans. While using water to extinguish electrical equipment, there is a possibility that the person doing it might get electrocuted. Water is a good conductor of electricity. It allows the passage of electricity easily. Electrocution might be fatal.
Ques. Why is LPG a better fuel than wood. [2 marks]
Ans. LPG being a gaseous fuel does not leave behind residue after burning. It is easy to store and transport. It has a lower ignition temperature than wood. It does not release smoke while burning whereas wood releases a large amount of smoke as a pollutant. It burns very easily and quickly than wood. Thus LPG proves to be a better fuel than wood.
Ques. Why does paper by itself catches fire but paper wrapped around an aluminium pipe does not? [2 marks]
Ans. When exposed to heat, paper catches fire immediately whereas the paper wrapped in an aluminium pipe does not because the aluminium pipe being a good conductor of heat absorbs the heat and does not let the paper wrapped along with it to burn. As the heat is absorbed by the aluminium pipe, the paper will not reach its ignition temperature thus it will not burn.
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