Flames and Combustion (Class 8)

Cheatsheet Content

1. Combustion Definition: A chemical process in which a substance reacts with oxygen to give off heat and light. Combustible Substances: Substances that burn in air or oxygen (e.g., wood, coal, LPG). Non-combustible Substances: Substances that do not burn (e.g., glass, stone). Conditions for Combustion: Presence of a combustible substance. Presence of oxygen (supporter of combustion). Attainment of ignition temperature. 2. Ignition Temperature Definition: The lowest temperature at which a substance catches fire and starts burning. Examples: Kerosene oil has a lower ignition temperature than wood. Role in Fire Prevention: Substances won't catch fire if their temperature is below their ignition temperature. 3. Inflammable Substances Definition: Substances that have very low ignition temperature and can easily catch fire with a flame (e.g., petrol, alcohol, LPG). They are highly volatile and dangerous. 4. Types of Combustion Rapid Combustion: Occurs quickly with the production of heat and light. Example: Burning of LPG, burning of a matchstick. Spontaneous Combustion: Combustion that occurs suddenly without the application of external heat. Example: Forest fires, combustion of white phosphorus at room temperature. Explosion: A very rapid combustion reaction producing a large amount of heat, light, and sound. Often accompanied by the release of gas. Example: Bursting of firecrackers. 5. Fire Control Methods of fire control: Removing one or more of the conditions necessary for combustion. Removing the fuel: Often not practical (e.g., a burning building). Cutting off air supply (oxygen): Using inert gases like carbon dioxide ($\text{CO}_2$). Covering with a blanket. Lowering the temperature below ignition temperature: Spraying water (most common method). Water cools the combustible material and also cuts off air supply by forming vapor. Fire Extinguishers: Water: Effective for fires involving ordinary combustible materials (wood, paper). Not for electrical fires or oil fires (water conducts electricity, oil floats on water). Carbon Dioxide ($\text{CO}_2$): Best for electrical equipment and inflammable materials (petrol, oil). It is heavier than oxygen, covers the fire like a blanket, and does not harm electrical equipment. Foam: Used for fires involving flammable liquids. 6. Flame Definition: The visible, gaseous part of a fire. Substances that produce flame: Those that vaporize during burning (e.g., kerosene, wax, LPG). Substances that do not produce flame: Those that do not vaporize (e.g., coal, charcoal). 7. Structure of a Candle Flame A candle flame has three main zones: Outermost Zone (Non-luminous Zone): Hottest part of the flame. Complete combustion of fuel occurs here due to ample oxygen supply. Blue in color. Used by goldsmiths for melting gold and silver. Middle Zone (Luminous Zone): Moderately hot. Partial combustion of fuel occurs here due to insufficient oxygen. Yellow in color, glows brightly. Produces unburnt carbon particles (soot). Innermost Zone (Dark Zone): Least hot part of the flame. Consists of unburnt wax vapors. Black in color. 8. Fuel Definition: Substances that produce heat and light on burning. Types of Fuel: Solid: Wood, coal, charcoal. Liquid: Kerosene, petrol, diesel. Gaseous: LPG, CNG, natural gas, biogas. 9. Characteristics of an Ideal Fuel Readily available. Cheap. Burns easily in air at a moderate rate. Produces a large amount of heat. Does not leave behind any undesirable substances. Has a low ignition temperature. Easy to transport and store. 10. Calorific Value of Fuel Definition: The amount of heat energy produced on complete combustion of 1 kg of a fuel. Unit: Kilojoule per kilogram ($\text{kJ/kg}$). Higher calorific value means more energy released per unit mass. Examples: Hydrogen ($\approx 150000 \text{ kJ/kg}$), LPG ($\approx 55000 \text{ kJ/kg}$), Wood ($\approx 17000 \text{ kJ/kg}$). 11. Harmful Effects of Burning Fuels Carbon Monoxide ($\text{CO}$): Produced by incomplete combustion of carbon fuels. It is a poisonous gas. Carbon Dioxide ($\text{CO}_2$): Released from complete combustion. Excess $\text{CO}_2$ in the atmosphere leads to global warming. Sulphur Dioxide ($\text{SO}_2$) and Nitrogen Oxides ($\text{NO}_x$): Released from burning coal, diesel, petrol. Cause acid rain and respiratory problems. Particulate Matter (Soot): Causes respiratory diseases and reduces visibility.