Class 8 Science Notes Chapter 6 (Combustion and Flame) – Science Book

Dear Students, we've prepared detailed notes for Chapter 6: Combustion and Flame from the NCERT Class 8 Science textbook, focusing on aspects relevant for your exam preparation.

Chapter 6: Combustion and Flame - Detailed Notes

1. Combustion

• Definition: Combustion is a chemical process in which a substance reacts rapidly with oxygen (usually from the air) to produce heat and light.
  • The substance undergoing combustion is called a combustible substance or fuel.
  • Oxygen is essential for combustion; it is called the supporter of combustion.
  • Combustion is an exothermic reaction (releases heat).
• Combustible Substances: Substances that burn in air to produce heat and light.
  • Examples: Wood, paper, kerosene, LPG (Liquefied Petroleum Gas), CNG (Compressed Natural Gas), coal, charcoal, magnesium ribbon, cow dung cakes.
• Non-Combustible Substances: Substances that do not burn in air.
  • Examples: Stone, glass, iron nails, sand, water, cement.
• Fuel:
  • Can be solid (e.g., coal, wood), liquid (e.g., petrol, kerosene), or gas (e.g., LPG, CNG, hydrogen).
  • Light is also produced during combustion, either as a flame or as a glow.

2. Conditions Necessary for Combustion

For combustion to occur, three conditions must be met simultaneously:

• (a) Presence of a Combustible Substance (Fuel): There must be something to burn.
• (b) Presence of a Supporter of Combustion (Oxygen/Air): Sufficient oxygen supply is required. (Activity: Candle covered with a glass tumbler extinguishes).
• (c) Attainment of Ignition Temperature: The fuel must be heated to its minimum temperature at which it catches fire.
  • Ignition Temperature: The lowest temperature at which a substance catches fire and starts burning.
  • Different substances have different ignition temperatures. (e.g., White phosphorus has a very low ignition temperature, ~30°C, catching fire spontaneously in air; wood needs to be heated significantly).
  • Substances with very low ignition temperatures that catch fire easily with a flame are called inflammable substances. Examples: Petrol, alcohol, LPG, CNG.

3. How Do We Control Fire?

Fire control involves removing one or more of the three essential conditions for combustion.

• (a) Removing the Fuel: If possible, remove the combustible material. (Difficult in large fires).
• (b) Cutting off the Air Supply (Oxygen):
  • Using Carbon Dioxide (CO2): CO2 is heavier than oxygen and forms a blanket around the fire, cutting off oxygen supply. It also cools the fuel. Commonly used in fire extinguishers. Excellent for electrical equipment and flammable liquids (like oil, petrol) as CO2 is non-conducting and doesn't spread the oil.
  • Using Water: Water cools the combustible material below its ignition temperature and water vapour also surrounds the fuel, cutting off air supply.
    • Caution: Water should not be used for fires involving electrical equipment (water conducts electricity) or oils/petrol (oil floats on water and continues to burn, spreading the fire).
  • Using Sand or Soil: Can smother small fires by cutting off air supply.
  • Using a Blanket: Wrapping a person whose clothes caught fire cuts off air supply.
• (c) Lowering the Temperature: Bringing the temperature of the fuel below its ignition temperature.
  • Water is the most common agent used for cooling (except for electrical/oil fires).

4. Types of Combustion

• (a) Rapid Combustion: Combustion that occurs quickly, producing heat and light.
  • Examples: Burning of LPG in a gas stove, burning of a matchstick, burning of kerosene oil. Requires external heat to start (like a spark or flame).
• (b) Spontaneous Combustion: Combustion in which a substance bursts into flames suddenly without the application of any apparent external cause (like heating or spark). Often occurs due to self-heating.
  • Examples: Phosphorus burning spontaneously at room temperature, spontaneous fires in coal mines due to coal dust, forest fires (sometimes due to heat of the sun or lightning, sometimes spontaneous).
• (c) Explosion: Combustion that occurs very suddenly (instantaneously) with the evolution of a large amount of heat, light, and sound, accompanied by the liberation of a large amount of gas. Often caused by pressure build-up.
  • Examples: Bursting of firecrackers, dynamite explosion.

5. Flame

• Definition: A flame is the visible, gaseous part of a fire resulting from highly exothermic combustion reactions taking place in a thin zone.
• Substances that Produce Flame: Substances that vaporize during burning produce flames.
  • Examples: Kerosene oil, molten wax (vaporizes through the wick), LPG, camphor.
• Substances that Do Not Produce Flame: Substances that do not vaporize on burning do not produce a flame; they may just glow.
  • Examples: Charcoal, coal (unless volatile matter is present).
• Structure of a Candle Flame: A candle flame typically has three distinct zones:
  • (i) Innermost Zone (Dark Zone):
    • Contains unburnt wax vapours.
    • Least hot part.
    • Black in colour due to lack of oxygen and presence of unburnt carbon particles.
  • (ii) Middle Zone (Luminous Zone):
    • Region of incomplete combustion (insufficient oxygen).
    • Yellow in colour.
    • Moderately hot.
    • Produces soot (unburnt carbon particles) which glow yellow, making the flame luminous.
  • (iii) Outermost Zone (Non-Luminous Zone):
    • Region of complete combustion (ample oxygen available).
    • Blue in colour (often faint).
    • Hottest part of the flame.
    • This is the zone used by goldsmiths for melting gold and silver because it is the hottest part.

6. Fuel and Fuel Efficiency

• Fuel: Any substance that is burnt to produce heat energy for domestic or industrial purposes.
• Characteristics of a Good (or Ideal) Fuel: (Note: No fuel is truly ideal)
  • Readily available.
  • Cheap.
  • Burns easily in air at a moderate rate.
  • Produces a large amount of heat (high calorific value).
  • Does not leave behind any undesirable substances or residues (low ash content).
  • Easy to handle, store, and transport.
  • Has a convenient ignition temperature (not too low, not too high).
  • Causes minimal pollution.
• Fuel Efficiency - Calorific Value:
  • Definition: The amount of heat energy produced on complete combustion of 1 kilogram (kg) of a fuel.
  • Unit: Kilojoules per kilogram (kJ/kg).
  • Higher the calorific value, the more efficient the fuel.
  • Approximate Calorific Values (kJ/kg):
    • Cow dung cake: 6,000 - 8,000
    • Wood: 17,000 - 22,000
    • Coal: 25,000 - 33,000
    • Petrol/Kerosene/Diesel: ~45,000
    • Methane/CNG/LPG: ~50,000 - 55,000
    • Biogas: 35,000 - 40,000
    • Hydrogen: ~1,50,000 (Highest calorific value, but storage/transport challenges and explosive nature limit its use as a common fuel).

7. Harmful Effects of Burning Fuels

Burning fuels releases various products into the environment, many of which are harmful.

• (a) Respiratory Problems: Burning carbon-based fuels like wood, coal, petroleum releases unburnt carbon particles (soot). These fine particles are dangerous pollutants causing respiratory diseases like asthma.
• (b) Carbon Monoxide (CO) Poisoning: Incomplete combustion (due to insufficient oxygen) of carbon fuels produces Carbon Monoxide (CO) gas instead of Carbon Dioxide (CO2).
  • CO is a very poisonous gas.
  • It binds strongly with haemoglobin in the blood, reducing the oxygen-carrying capacity, which can be fatal. (Reason why sleeping in a closed room with burning coal is dangerous).
• (c) Global Warming: Combustion of most fuels releases Carbon Dioxide (CO2). While essential for plants, excess CO2 in the atmosphere traps heat radiation, leading to an increase in the average temperature of the Earth's atmosphere. This is called Global Warming.
  • Consequences: Melting of polar glaciers, rise in sea level, coastal flooding, changes in climate patterns.
• (d) Acid Rain: Burning of coal and diesel releases Sulphur Dioxide (SO2) gas. Petrol engines release gaseous Nitrogen Oxides (NOx).
  • These oxides (SO2 and NOx) dissolve in rainwater to form acids (Sulphuric acid and Nitric acid).
  • Rain containing these acids is called Acid Rain.
  • Consequences: Harmful for crops, damages buildings and monuments (like Taj Mahal - marble corrosion), makes soil acidic, harms aquatic life.
• (e) Deforestation: Using wood as fuel on a large scale leads to cutting down of trees, causing deforestation.

Key Focus Areas for Exams:

• Definition of Combustion.
• Essential conditions for combustion (Fuel, Oxygen, Ignition Temperature).
• Definition and examples of Inflammable substances.
• Methods of fire control and the principle behind each (removing fuel, cutting off air, lowering temperature).
• Why water is not used for electrical and oil fires.
• Role of CO2 as a fire extinguisher.
• Types of combustion (Rapid, Spontaneous, Explosion) with examples.
• Why some substances produce flames (vaporization).
• Structure of a candle flame (zones, colours, relative temperature, hottest part, completeness of combustion in each zone).
• Definition and unit of Calorific Value.
• Relative comparison of calorific values of common fuels (e.g., Wood vs LPG vs Hydrogen).
• Harmful products of burning fuels (CO, CO2, SO2, NOx, unburnt carbon) and their specific effects (Poisoning, Global Warming, Acid Rain, Respiratory diseases).

This comprehensive breakdown covers the core concepts of the chapter relevant for competitive government exams based on the NCERT syllabus. Remember to correlate these points with diagrams and activities mentioned in the textbook for better understanding.