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Combustion and Flame - Types of Combustion: Rapid, Spontaneous, and Explosion

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 light.

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Rapid Combustion: A type of combustion in which a substance burns rapidly and produces heat and light. Examples include the burning of LPGLPG (Liquefied Petroleum Gas) or a matchstick.

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Spontaneous Combustion: A type of combustion where a material suddenly bursts into flames without the application of any apparent cause or external heat. For example, White Phosphorus (P4P_4) catches fire spontaneously at room temperature (around 35∘C35^\circ C).

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Explosion: A sudden reaction that takes place with the evolution of heat, light, and sound. A large amount of gas is evolved in the reaction. Example: Ignition of firecrackers containing substances like Potassium Nitrate (KNO3KNO_3) and Sulfur (SS).

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Ignition Temperature: The lowest temperature at which a substance catches fire. Spontaneous combustion occurs when the ignition temperature is reached through slow internal oxidation or ambient heat.

📐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}

P4+5O2→P4O10 (Spontaneous oxidation of Phosphorus)P_4 + 5O_2 \rightarrow P_4O_{10} \text{ (Spontaneous oxidation of Phosphorus)}

2Mg+O2→2MgO+Light+Heat2Mg + O_2 \rightarrow 2MgO + \text{Light} + \text{Heat}

💡Examples

Problem 1:

A heap of coal dust in a mine suddenly starts burning without any matchstick being struck. Identify the type of combustion and explain why it happened.

Solution:

This is an example of Spontaneous Combustion.

Explanation:

Coal dust has a tendency to undergo slow oxidation. In poorly ventilated mines, the heat generated by this oxidation cannot escape, raising the temperature of the coal dust to its ignition temperature, causing it to burst into flames without external heating.

Problem 2:

When a gas stove knob is turned on and a lighter is clicked, the LPGLPG burns immediately. Which type of combustion does this represent?

Solution:

This represents Rapid Combustion.

Explanation:

Substances like LPGLPG (consisting mainly of Propane C3H8C_3H_8 and Butane C4H10C_4H_{10}) have low ignition temperatures and react quickly with Oxygen (O2O_2) once a small spark provides the initial energy, producing significant heat and light instantly.