Quick Revision Notes & Formula Sheet

Class 8 Science - Chapter: Combustion and Flame

Combustion (दहन): A chemical process in which a substance reacts with oxygen to give off heat and light. The substance that undergoes combustion is said to be combustible (दहनशील पदार्थ).
Non-Combustible Substances (अदहनशील पदार्थ): Substances that do not burn in air (e.g., stone, glass, water).
Ignition Temperature (ज्वलन ताप): The lowest temperature at which a combustible substance catches fire. A substance cannot catch fire if its temperature is lower than its ignition temperature.
Flammable Substances (ज्वलनशील पदार्थ): Substances which have very low ignition temperatures and can easily catch fire with a flame (e.g., Petrol, Alcohol, LPG).
Flame (ज्वाला): A zone of burning gases. Substances that vapourise during burning produce flames (e.g.,蜡烛/Wax, Kerosene oil).
Fuel (इंधन): The source of heat energy for domestic and industrial purposes (e.g., Wood, Coal, Petrol, LPG). Good fuels are cheap, readily available, burn easily, and have a high calorific value.

For combustion to take place, three essential conditions are required (often called the Fire Triangle):

Presence of a Combustible Substance: Fuel must be present.
Supporter of Combustion: Oxygen (air) must be available.
Attainment of Ignition Temperature: The temperature of the fuel must be raised to or above its ignition temperature.

Rapid Combustion (तीव्र दहन): Combustion in which gas burns rapidly and produces heat and light with the help of a spark or flame (e.g., Burning of LPG, burning of matchstick).
Spontaneous Combustion (स्वतः दहन): Combustion in which a material suddenly bursts into flames, without the application of any apparent cause (e.g., Phosphorus burning at room temperature, forest fires due to sun heat).
Explosion (विस्फोट): A sudden reaction that takes place with the evolution of heat, light, and sound, accompanied by the release of a large amount of gas (e.g., Crackers bursting).

A flame has three main zones:

Outer Zone (बाह्य क्षेत्र):
- Blue in color.
- Zone of complete combustion.
- Hottest part of the flame.
- Used by goldsmiths to melt gold and silver.
Middle Zone (मध्य क्षेत्र):
- Yellow in color.
- Zone of partial combustion.
- Moderately hot.
Innermost Zone (आंतरिक क्षेत्र):
- Black in color.
- Contains unburnt wax vapors.
- Least hot part of the flame.

Fire can be controlled by removing one or more requirements of combustion:

Water: Commonly used to cool down the combustible material below its ignition temperature. (Note: Water is not suitable for fires involving electrical equipment or oil.)
Carbon Dioxide ($CO_2$): The best extinguisher for fires involving electrical equipment and inflammable materials like petrol. $CO_2$ is heavier than oxygen and covers the fire like a blanket, cutting off the contact between oxygen and fuel.

Definition: The amount of heat energy produced on complete combustion of 1 kg of a fuel.
Unit: Kilojoule per kg (kJ/kg).
Formula / Relation: $$\text{Calorific Value} = \frac{\text{Total Heat Produced (in kJ)}}{\text{Mass of Fuel Burnt (in kg)}}$$

Carbon Fuels (Wood, Coal, Petroleum): Release unburnt carbon particles causing respiratory diseases like asthma.
Incomplete Combustion: Produces Carbon Monoxide ($CO$) gas, which is a very poisonous, suffocating gas that can kill a person sleeping in a room with burning coal.
Global Warming: Increased concentration of Carbon dioxide ($CO_2$) in the air leads to global warming (rise in temperature of the earth's atmosphere).
Acid Rain: Oxides of sulphur and nitrogen released by burning coal and diesel dissolve in rain water and form acids, damaging buildings, crops, and soil.