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Combustion and Flame - Conditions Necessary for Combustion

Grade 8CBSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Combustion is a chemical process in which a substance reacts with oxygen (O2O_2) to give off heat and sometimes light.

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Substances that undergo combustion are called combustible substances or fuels. Examples include wood, coal, and LPGLPG.

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Air is essential for combustion because it provides oxygen (O2O_2), which acts as a supporter of combustion.

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The lowest temperature at which a substance catches fire is called its ignition temperature. A substance will not burn if its temperature is lower than this value.

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Inflammable substances are those which have very low ignition temperatures and can easily catch fire with a flame. Examples include petrol, alcohol, and Liquefied Petroleum Gas (LPGLPG).

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The three essential requirements for producing fire are: 1. Fuel, 2. Air (to supply oxygen), and 3. Heat (to raise the temperature of the fuel beyond the ignition temperature).

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Fire can be controlled by removing one or more of these requirements, such as using water to lower the temperature or using CO2CO_2 to cut off the supply of O2O_2.

📐Formulae

Fuel+O2→CO2+H2O+Heat+LightFuel + O_2 \rightarrow CO_2 + H_2O + \text{Heat} + \text{Light}

CH4+2O2→CO2+2H2O+HeatCH_4 + 2O_2 \rightarrow CO_2 + 2H_2O + \text{Heat}

C+O2→CO2+HeatC + O_2 \rightarrow CO_2 + \text{Heat}

💡Examples

Problem 1:

Why does a piece of paper burn quickly, but a piece of paper wrapped around an aluminum pipe does not ignite easily?

Solution:

The heat supplied to the paper wrapped around the aluminum pipe is transferred to the metal by conduction.

Explanation:

Aluminum is a good conductor of heat. It carries the heat away from the paper, preventing the paper from reaching its ignition temperature. Therefore, the paper does not burn.

Problem 2:

Explain why water is not used to control fires involving electrical equipment or oil.

Solution:

Water conducts electricity and is heavier than oil, making it ineffective and dangerous in these scenarios.

Explanation:

For electrical fires, water may conduct electricity and give an electric shock to the person spraying it. For oil fires, oil is lighter than water and floats on top, so the fire continues to burn on the surface of the water. Carbon dioxide (CO2CO_2) is the preferred extinguisher as it is heavier than O2O_2 and blankets the fire.

Problem 3:

A candle is placed in a glass jar and the jar is covered with a lid. What happens to the flame after some time?

Solution:

The flame flickers and eventually goes out.

Explanation:

Combustion requires a continuous supply of oxygen (O2O_2). Once the lid is closed, the limited amount of O2O_2 inside the jar is consumed and converted into CO2CO_2. Without oxygen to support combustion, the chemical reaction stops and the flame is extinguished.