8 Science Chapter 6 Combustion And Flame

Chapter Notes and Summary
• A chemical process in which a substance reacts with oxygen to give off heat is called combustion. substance that undergoes combustion is said to be combustible. It is also known as fuel.
Example : When a magnesium ribbon burns, it combines with oxygen of air forming magnesium oxide and produces heat and light.
Magnesium + Oxygen → Magnesium oxide + Heat + Light
Combustible and Non-Combustible substances :
• substances which burn in air or oxygen are called combustible substances. e.g., Petrol, LPG, Kerosene etc.
• substances which do not burn in air or oxygen are called non-combustible substances e.g., water, glass, sand etc.
Types of combustion :
Rapid combustion : When combustion occurs rapidly, it is called rapid combustion.
Spontaneous combustion : type of combustion in which material suddenly bursts into flames, without application of any apparent cause is called spontaneous combustion eg.; Burning of Phosphorous.
Explosion : When a sudden reaction takes place with release of heat and light, and evolution of large amount of gas takes place, it is called explosion e.g., fire crackers.
Necessary conditions for combustion : There are three necessary conditions for combustion. These are :
• Presence of a combustible substance.
• Presence of a supporter of combustion.
• Heating combustible substance to its ignition temperature.
Ignition Temperature : lowest temperature at which a substance catches fire and starts burning is called its ignition temperature. A combustible substance cannot catch fire if its temperature is lower than its ignition temperature. e.g., burning of matchstick.
Matchstick : A mixture of antimony trisulphide, potassium chlorate and white phosphorous with some glue and struck was applied on head of a match made of suitable wood. When struck against a rough surface, white phosphorous got ignited due to heat of friction. These days head of safety match contains only antimony trisulphide and potassium chlorate.
• When matchstick is struck against rubbing surface, some red phosphorous gets converted into white phosphorous.
Fire extinguishers : A substance that disrupts contact between air and fire is called fire extinguisher.
• Fire brigade pours water on fire. Water cools combustible material so that its temperature is brought below its ignition temperature. This prevents fire from spreading.
• For fire involving electrical equipments and inflammable materials like petrol, carbon dioxide is best extinguisher. CO2 being heavier than oxygen, covers fire like a blanket, and fire is controlled.
Flame : A flame is a region where combustion of gaseous substance or vapour takes place.
Zones of Flames :
Black Zone : Innermost zone of unburnt vapours.
Outer zone of complete combustion (blue) Middle zone of partial combustion
(yellow) innermost zone of unburnt wax vapours (black) Hottest part Moderately hot least hot wax candle Different Zones of Candle Flame l Bright and Luminous Zone : It is middle zone, brightness of this zone is due to glow of unburnt carbon particles.
Blue Zone : It is outer zone of complete combustion. It is hottest and non-luminous zone.
Ideal or Good Fuel :
• Readily available l Cheap l It burns easily in air at a moderate rate. l It produces large amount of heat.
• It does not leave behind any undesirable substances.
Calorific Value : amount of heat energy produced on complete combustion of 1 kg of fuel is called its calorific value. Its unit is kilojoule per kg (kj/kg).
Burning of fuels leads to harmful products :
• Carbon fuel such as wood, coal and petroleum releases unburnt carbon particles. These fine particles are dangerous pollutants causing respiratory diseases such as asthma.
• Incomplete combustion of these fuels give ‘CO’ i.e., carbon monoxide gas. It is a very poisonous gas.
• Combustion of most fuels releases carbon dioxide in environment, increased concentration of carbon dioxide in air is believed to cause global warming.
• Burning of coal and diesel releases sulphur dioxide gas. These oxides dissolve in rain water and form acids. Such rain is called acid rain.

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