Chemistry - Combustion and Flame



  • A chemical process in which a substance reacts with oxygen and give off heat is known as combustion.

  • The substance that undergoes combustion is called as combustible or fuel.

  • The fuel can be in the form of solid, liquid, or gas.

  • During the combustion, light is also given off either in the form of a flame or as a glow.

Flame, Glow
  • The substances which vaporize during burning time, give flames.

  • There are three different zones of a flame dark zone, luminous zone and non-luminous zone.

Different Zone of Flame
  • Different substances catch fire at different temperatures.

  • The lowest temperature at which a substance catches fire is known as its ignition temperature.

  • A match contains antimony trisulphide and potassium chlorate.

  • The rubbing surface of match contains powdered glass and a little red phosphorus.

  • Red phosphorus is much less dangerous.

  • When the match stick is struck against the rubbing surface, some red phosphorus gets converted into white phosphorus; the process immediately reacts with potassium chlorate present in the matchstick head and produce enough heat to ignite antimony trisulphide; likewise, combustion starts.

  • The substances, which have very low ignition temperature and can easily catch fire with a flame, are known as inflammable substances. E. g. petrol, alcohol, Liquified Petroleum Gas (LPG), etc.

Fire Extinguisher

  • Water is the most common fire extinguisher.

  • Water, as fire extinguisher, works only when things like wood and paper are on fire.

Firemen Extinguishing Fire
  • If electrical equipment is on fire, water may conduct electricity and damage those trying to douse the fire.

  • Water is also not a good extinguisher for fires involving oil and petrol.

  • For fires that involve electrical equipment and inflammable materials such as petrol, Carbon Dioxide (CO2) is the best extinguisher.

Fire Extinguisher
  • One of the ways to get CO2 is to release plenty of dry powder of chemicals such as sodium bicarbonate (baking soda) or potassium bicarbonate.

  • Phosphorus burns in air at room temperature.

  • The amount of heat energy produced on complete combustion of 1 kg of a fuel is known as its calorific value.

  • The calorific value of a fuel is measured in a unit called kilojoule per kg (kJ/kg).

  • The following table illustrates the Calorific Values of Different Fuels −

Fuel Calorific Value (kJ/kg)
Cow dung cake 6000-8000
Wood 17000-22000
Coal 25000-33000
Petrol 45000
Kerosene 45000
Diesel 45000
Methane 50000
CNG 50000
LPG 55000
Biogas 35000-40000
Hydrogen 150000
  • Combustion of most fuels releases carbon dioxide in the environment.

  • Increased concentration of carbon dioxide in the air is most likely causes global warming.

  • The rise in temperature of the atmosphere of the earth is known as Global Warming.

  • Global warming causes melting of polar glaciers, which leads to a rise in the sea level that ultimately causing floods in the coastal regions.

  • Oxides of sulphur and nitrogen dissolve in rain water and form acids; such type of rain is known as acid rain.