Combustion and Flame

Combustion and Flame 


What is Combustion? 


  • Combustion is a chemical process in which a substance reacts with oxygen, producing heat and often light. 
  • The substance undergoing combustion is termed combustible and can exist in solid, liquid, or gas forms. 
  • Examples include magnesium charcoal, both of which burn to form magnesium oxide and carbon dioxide, respectively.


Combustible Substances: 

  • Combustible substances participate in combustion reactions. 
  • These substances are also known as fuels and release energy in the form of heat and light when they react with oxygen. 
  • Solid examples include charcoal and magnesium, while liquid and gas fuels are also common.


Non-Combustible Substances:


  • Non-combustible substances do not undergo combustion and do not react with oxygen to produce heat. 
  • Examples encompass materials that are resistant to burning, such as certain metals or inert substances. 
  • Non-combustible materials are often used in applications where resistance to high temperatures is essential.


Requirements for Combustion: 


  • Combustible Substance:

The primary requirement for combustion is a substance capable of undergoing a chemical reaction with oxygen. This substance, known as the combustible material or fuel, provides the source of energy during combustion. 

  • Oxygen:

Combustion necessitates the presence of oxygen as a reactant. The combustible substance reacts with oxygen to form products like carbon dioxide and water in the case of hydrocarbons. 

  • Ignition temperature:

The lowest temperature at which a substance catches fire is its ignition temperature. Cooking oil, kerosene oil, and wood won't catch fire at room temperature but can ignite when heated, emphasizing the importance of handling flammable substances cautiously. 

An ignition source is also essential to initiate the combustion process. This can be in the form of heat, a spark, or an open flame, depending on the specific properties of the combustible material. 




Role of Air in Combustion: 

  • Air is necessary for combustion to occur. 
  • Adequate air supply supports efficient combustion, producing heat and light. 
  • Lack of air can lead to incomplete combustion, characterized by flickering flames, smoke, or the extinguishing of the flame.


Complete Combustion: 

  • In complete combustion, a fuel burns in the presence of sufficient oxygen. 
  • It results in the production of carbon dioxide and water vapor as byproducts. 
  • It is a clean and efficient process. 
  • Maximum heat energy is released in this. 
  • Ideal for energy production.


Incomplete Combustion: 

  • In incomplete combustion, a fuel burns in a limited supply of oxygen. 
  • It leads to the formation of products such as carbon monoxide, unburnt carbon particles, and water vapor. 
  • It's a less efficient process. 
  • Produces harmful byproducts like carbon monoxide and particulate matter.


Inflammable Substances: 

  • Inflammable substances are those with a very low ignition temperature, making them easily catch fire when exposed to a flame. 
  • Examples of inflammable substances include petrol, alcohol, and Liquified Petroleum Gas (LPG). 
  • These substances pose a fire hazard and require careful handling and storage to prevent accidents.


History of Matchsticks: 

  • Matchsticks have a long history, dating back over five thousand years to ancient Egypt. 
  • Early matches were small pieces of pinewood dipped in sulphur.The modern safety match, as we know it, was developed about two hundred years ago. 
  • Old Match Composition:

- Ancient matches used a mixture of antimony trisulphide, potassium chlorate, and white phosphorus. 

- The match head was made of suitable wood, and when struck on a rough surface, white phosphorus ignited due to friction heat, initiating combustion. 

- However, white phosphorus was hazardous for both manufacturing workers and users. 

  • Modern Safety Matches:

- Today's safety match heads contain only antimony trisulphide and potassium chlorate. 

- The rubbing surface includes powdered glass and a safer form of phosphorus called red phosphorus. 

- When the match is struck, red phosphorus converts to white phosphorus, reacting with potassium chlorate to generate enough heat. 

- This heat ignites antimony trisulphide, starting the combustion process. 

  • Safety Improvements:

- The removal of white phosphorus contributes to the safety of both match manufacturers and users. 

- Red phosphorus is less dangerous, making modern matches a safer alternative.


Forest Fires: 

  • Forest fires are uncontrolled and rapid fires that spread through wooded areas, particularly during dry and hot conditions. 
  • They often start from sources like lightning, human activities, or even spontaneous combustion. 
  • Dry grasses catching fire can lead to the rapid spread of flames, engulfing trees and turning into a large-scale forest fire. 
  • Controlling forest fires is challenging due to their intensity and the abundance of fuel (vegetation) in the forest. 
  • In extreme cases, these fires can cause significant environmental damage, loss of wildlife habitat, and pose threats to human lives and property. 
  • Prevention measures, early detection, and efficient firefighting techniques are crucial for minimizing the impact of forest fires.


How Do We Control Fire? 

Controlling Fire: 

  • Fire accidents in homes, shops, and factories are not uncommon. 
  • Fire Brigade's Role: 

- Cities/towns often have fire brigade stations to respond to fire emergencies. 

- When a fire brigade arrives, it pours water on the fire. Water serves to cool the combustible material, bringing its temperature below the ignition point. 

- Water vapours created also help cut off the air supply, contributing to extinguishing the fire. 

  • Three Essential Requirements for Fire: 

- Fire requires three essential elements: fuel, air (supplying oxygen), and heat (raising the fuel's temperature beyond ignition). 

- To control fire, one or more of these requirements must be removed. 

  • Fire Extinguishers: 

- Water is a common fire extinguisher effective for wood and paper fires but not suitable for electrical or oil-based fires. 

- Water may conduct electricity and can be less effective for oil fires as it sinks below the oil, allowing it to keep burning. 

  • Carbon Dioxide (CO2) Extinguisher: 

- For electrical and inflammable material fires (e.g., petrol), CO2 is the preferred extinguisher. 

- CO2, being heavier than oxygen, blankets the fire, cutting off contact between fuel and oxygen, thereby controlling the fire. 

- CO2 is advantageous as it typically does not harm electrical equipment. 

  • Supply of Carbon Dioxide: 

- Carbon dioxide can be stored at high pressure as a liquid in cylinders, similar to how LPG is stored. 

- Upon release, CO2 expands, forming a blanket around the fire and reducing the fuel's temperature, making it an excellent fire extinguisher. 

- An alternative way to generate CO2 is by releasing dry powder chemicals like sodium bicarbonate (baking soda) or potassium bicarbonate near the fire, which then give off CO2. 

  • Fire Extinguisher's Function: 

- A fire extinguisher's role is to cut off the air supply, lower the temperature of the fuel, or both. 

- Eliminating fuel entirely is often impractical, especially in cases like a building fire where the entire structure acts as the fuel. 

  • Different types of fire extinguishers are suitable for specific fire types. 
  • Understanding the nature of the fire is crucial in selecting the appropriate extinguisher to ensure effective fire control without causing harm.


Types of Combustion 

  • Rapid Combustion: 

- When a burning matchstick or gas lighter is brought near a gas stove, the gas burns rapidly, producing heat and light. 

- This type of combustion, where a substance burns quickly, is known as rapid combustion.




  • Combustion at Room Temperature:

- Some substances, like phosphorus, can burn in air at room temperature. 

  • Spontaneous Combustion: 

- Spontaneous combustion occurs when a material suddenly bursts into flames without any apparent external cause. 

- Examples include spontaneous combustion of coal dust in coal mines or forest fires due to natural factors like sun heat or lightning. Human carelessness often contributes to forest fires. 




  • Explosion: 

- Fireworks during festivals involve a sudden reaction, releasing heat, light, and sound. A significant amount of gas is formed in the process. 

- Such rapid reactions with the liberation of gas are termed explosions. 

- Pressure applied to a cracker can also cause an explosion. 





  • A flame is the visible, gaseous part of a fire, typically characterized by its color, heat, and light. 
  • Flame colors vary depending on the material burning. For example, LPG flames are blue, candle flames are yellow, and burning magnesium produces a bright white flame. 




Structure of Flame 

Flame Zones: 

  • Hottest Part - Outer Zone of Complete Combustion (Blue):

- The outermost zone of complete combustion appears blue and is the hottest part of the flame. 

  • Moderately Hot - Middle Zone of Partial Combustion (Yellow):

- The middle zone of partial combustion is yellow and moderately hot. 

  • Least Hot - Innermost Zone of Unburnt Wax Vapours (Black):

- The innermost zone is black and least hot, containing unburnt wax vapours. 




Flame Formation: 

  • Substances that vaporize during burning give rise to flames. For example, kerosene oil and molten wax vaporize through the wick, forming flames. 
  • Charcoal does not vaporize and, therefore, does not produce a flame. 
  • Observation with Glass Plate/Slide: 

- When a clean glass plate/slide is introduced into the luminous zone of the flame, a circular blackish ring forms, indicating the deposition of unburnt carbon particles. 

  • Copper Wire Experiment: 

- Holding a thin copper wire just inside the flame for about 30 seconds results in the portion outside the flame becoming red hot. 

- This indicates that the non-luminous zone of the flame has a high temperature, making it the hottest part. 

  • Goldsmiths' Technique: 

- Goldsmiths use the outermost zone of a flame, blown with a metallic blow-pipe, for melting gold and silver. 

- The outer zone provides the highest temperature, making it suitable for efficient metal melting. 


What is a Fuel?


  • Fuels are substances used as sources of heat energy for domestic and industrial purposes. Common fuels include wood, charcoal, petrol, kerosene, etc. 
  • Characteristics of a Good Fuel: 

- Readily Available: A good fuel should be easily accessible. 

- Affordable: It should be cost-effective. 

- Ease of Combustion: Burns easily in air at a moderate rate. 

- High Heat Production: Produces a substantial amount of heat. 

- Clean Combustion: Leaves behind no undesirable substances. 

  • Ideal Fuel Consideration: 

- While there might not be a perfect fuel, the aim is to find one that meets most requirements for a specific use. 

- Different fuels have varying costs, and some are more economical than others. 

  • List of Fuels:

- Solid Fuels: Wood, Charcoal, Coal.




- Liquid Fuels: Petrol, Kerosene, Diesel.




- Gaseous Fuels: Natural Gas (Methane),  Propane (LPG), Butane. 




Fuel Efficiency 

  • Scenario:

- Boiling a given quantity of water using cow dung, coal, and LPG as fuels. 

  • Fuel Preference:

- The preferred fuel would be LPG. 

  • Reason:

- LPG is likely to be more efficient due to its higher calorific value, meaning it produces more heat energy per kilogram during complete combustion. 

  • Heat Production:

- Not all fuels produce the same amount of heat. 

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

  • Calorific Value Unit:

- Calorific value is measured in kilojoules per kilogram (kJ/kg). 

  • The formula for calculating fuel efficiency is given by:

Fuel Efficiency = Output Energy/ Input Energy × 100 


Output Energy- Output Energy is the useful energy obtained from the fuel (e.g., heat, work). 

Input Energy- Input Energy is the total energy supplied by the fuel.




Harmful Effects of Burning Fuels: 

  • Unburnt Carbon Particles: 

- Carbon fuels like wood, coal, and petroleum release fine carbon particles. 

-These particles are dangerous pollutants causing respiratory diseases such as asthma. 

  • Carbon Monoxide Gas: 

- Incomplete combustion of fuels produces carbon monoxide gas. 

- Carbon monoxide is highly poisonous and can be lethal if produced in closed rooms (e.g., burning coal). 

  • Carbon Dioxide Emission: 

- Combustion of most fuels releases carbon dioxide into the environment. 

- Increased carbon dioxide concentration contributes to global warming, leading to the rise in Earth's temperature. 

- Global warming results in the melting of polar glaciers, causing a rise in sea levels and floods in coastal areas. 

  • Sulphur Dioxide and Nitrogen Oxides: 

- Burning coal and diesel releases sulphur dioxide gas, which is suffocating and corrosive. 

- Petrol engines emit gaseous oxides of nitrogen, contributing to air pollution. 

- Sulphur dioxide and nitrogen oxides dissolve in rainwater, forming acids, resulting in acid rain. 

- Acid rain is harmful to crops, buildings, and soil.


Transition to Cleaner Fuels:

  • To mitigate harmful effects, diesel and petrol in automobiles are being replaced by Compressed Natural Gas (CNG). 
  • CNG produces fewer harmful products, making it a cleaner fuel.