Class 8 Science Notes Chapter 6 (Chapter 6) – Examplar Problems Book

Alright class, let's focus on Chapter 6, 'Combustion and Flame' from your Science Exemplar book. This is a crucial chapter, not just for your school exams but also for various government competitive exams where basic science concepts are tested. Pay close attention as we break down the key ideas.

Chapter 6: Combustion and Flame - Detailed Notes for Competitive Exams

1. What is Combustion?

• Definition: Combustion is a chemical process where 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 a fuel. Examples: Wood, Paper, Kerosene, Petrol, LPG, CNG, Coal, Charcoal, Magnesium ribbon.
  • Substances that do not burn are called non-combustible substances. Examples: Stone, Glass, Iron nails, Sand, Water.
• Light Production: Light during combustion can be produced either as a flame or as a glow.
• Key Reactant: Oxygen is essential for combustion; it is called the supporter of combustion. In some specific cases, other substances like chlorine can also support combustion (e.g., sodium burning in chlorine).

2. Conditions Necessary for Combustion:

For combustion to occur, three conditions must be met simultaneously (often called the 'Fire Triangle'):
* (a) Presence of Fuel: A combustible substance must be present.
* (b) Presence of Air (Oxygen): A supporter of combustion (usually oxygen) must be available.
* (c) Attainment of Ignition Temperature: The fuel must be heated to its minimum ignition temperature.
* Ignition Temperature: The lowest temperature at which a substance catches fire and starts burning.
* Substances with low ignition temperatures catch fire easily.
* Substances with high ignition temperatures require significant heating to burn.

• Inflammable Substances: Substances that have a very low ignition temperature and can easily catch fire with a flame are called inflammable substances. Examples: Petrol, Alcohol, LPG, CNG, Perfumes (contain alcohol). Note: Inflammable and Flammable mean the same thing - easily catches fire.

3. How to Control Fire:

Fire control involves removing one or more of the three essential requirements for combustion.
* (a) Removing the Fuel: If possible, remove the combustible material. (Difficult in large fires).
* (b) Cutting off the Air Supply:
* Covering the fire with a blanket, sand, or soil.
* Using Carbon Dioxide (CO2): CO2 is heavier than oxygen and forms a layer around the burning substance, cutting off oxygen supply. It also cools the fuel. Commonly used in fire extinguishers. Excellent for electrical fires and fires involving inflammable liquids (like oil, petrol) where water cannot be used.
* (c) Lowering the Temperature:
* Using Water: Water cools the combustible material below its ignition temperature. Effective for fires involving wood, paper, etc.
* Caution: Water should not be used on electrical fires (risk of electrocution as water conducts electricity) or oil/petrol fires (water is heavier, sinks below the oil, and the oil continues to burn on top, potentially spreading the fire).

• Fire Extinguishers: Devices used to put out fires. Common types:
  • Soda-acid type (produces CO2 via chemical reaction).
  • Dry powder type (releases chemicals like sodium bicarbonate which decompose on heating to release CO2).
  • CO2 extinguishers (store CO2 under pressure).

4. Types of Combustion:

• (a) Rapid Combustion: Combustion that occurs quickly, producing heat and light in a short time. Example: Burning of LPG in a gas stove, burning a matchstick.
• (b) Spontaneous Combustion: Combustion in which a substance bursts into flames suddenly without the application of any apparent external heat. This often happens due to self-heating caused by slow oxidation. Examples: Phosphorus burning on its own at room temperature, coal dust fires in mines, forest fires due to heat of the sun or lightning.
• (c) Explosion: A very fast combustion reaction occurring with the evolution of a large amount of heat, light, and sound, accompanied by the rapid formation of gases. Example: Bursting of firecrackers, dynamite explosion.

5. Flame:

• Definition: A flame is the visible, gaseous region where the combustion of a fuel takes place.
• Formation: Substances that vaporise during burning produce flames. Examples: Wax, Kerosene, LPG, Magnesium.
• No Flame: Substances that do not vaporise on burning do not produce a flame, they just glow. Example: Charcoal, Coal (mostly glows).
• Structure of a Candle Flame: A candle flame typically has three distinct zones:
  • (i) Innermost Zone (Dark Zone): Contains unburnt wax vapours. No combustion occurs here due to lack of oxygen. It is blackish and the least hot part.
  • (ii) Middle Zone (Luminous Zone): Partial/Incomplete combustion occurs here due to limited oxygen supply. Contains hot, glowing unburnt carbon particles which make the flame yellow and luminous. It is moderately hot. Soot (carbon) is formed in this zone.
  • (iii) Outermost Zone (Non-luminous Zone): Complete combustion occurs here due to plentiful supply of oxygen. It is usually faint blue in colour and is the hottest part of the flame. It is non-luminous or faintly luminous.

6. Fuel and Fuel Efficiency:

• Fuel: Any substance burnt to produce heat energy for domestic or industrial purposes.
  • Types: Solid (Wood, Coal, Charcoal, Cow dung cakes), Liquid (Kerosene, Petrol, Diesel), Gaseous (LPG, CNG, Biogas, Hydrogen gas).
• Characteristics of an Ideal Fuel (or Good Fuel):
  • High Calorific Value (produces large amount of heat).
  • Burns at a moderate rate.
  • Readily available and cheap.
  • Easy to transport and store.
  • Does not produce harmful gases or leave much residue (ash) after burning.
  • Has a proper ignition temperature (not too low, not too high).
• 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).
  • Significance: Higher the calorific value, more efficient the fuel.
  • Examples (Approximate Values in kJ/kg):
    • Cow dung cake: 6,000 - 8,000
    • Wood: 17,000 - 22,000
    • Coal: 25,000 - 33,000
    • Petrol/Kerosene/Diesel: ~45,000
    • CNG/LPG: ~50,000 - 55,000
    • Biogas: 35,000 - 40,000
    • Hydrogen: ~1,50,000 (Highest calorific value)

7. Harmful Effects of Burning Fuels:

Burning fuels has significant environmental impacts:

• Air Pollution:
  • Unburnt Carbon Particles: Released from burning carbon-rich fuels like wood, coal, petroleum. Cause respiratory problems like asthma.
  • Carbon Monoxide (CO): Produced by incomplete combustion of fuels. It is a highly poisonous gas that reduces the oxygen-carrying capacity of blood, which can be fatal.
  • Carbon Dioxide (CO2): Produced by complete combustion. While essential for plants, excess CO2 in the atmosphere traps heat, leading to Global Warming (increase in Earth's average temperature).
  • Oxides of Sulfur (SO2) and Nitrogen (NOx): Released mainly from burning coal, diesel, and petrol. These are suffocating and corrosive gases. They dissolve in rainwater to form Acid Rain.
• Acid Rain: Rainwater containing dissolved sulfuric acid and nitric acid. It damages crops, soil, aquatic life, and buildings/monuments (like the Taj Mahal).
• Global Warming: The gradual increase in the overall temperature of the earth's atmosphere generally attributed to the greenhouse effect caused by increased levels of carbon dioxide and other pollutants. Leads to melting glaciers, rising sea levels, and climate change.

Using Cleaner Fuels: Fuels like LPG and CNG produce fewer pollutants compared to coal, wood, or petrol/diesel, hence they are considered cleaner fuels. Promoting renewable energy sources (solar, wind, hydro) is crucial to reduce reliance on fossil fuels.

Multiple Choice Questions (MCQs)

Here are 10 MCQs based on Chapter 6 for your practice:

1. Which of the following is a non-combustible substance?
   (a) Wood
   (b) Paper
   (c) Iron nail
   (d) Straw

2. The lowest temperature at which a substance catches fire is called its:
   (a) Melting point
   (b) Ignition temperature
   (c) Boiling point
   (d) Critical temperature

3. Which gas is essential for combustion?
   (a) Carbon dioxide
   (b) Nitrogen
   (c) Oxygen
   (d) Hydrogen

4. Which zone of a candle flame is the hottest?
   (a) Innermost zone (dark)
   (b) Middle zone (luminous)
   (c) Outermost zone (non-luminous)
   (d) All zones have the same temperature

5. Which of the following is an example of spontaneous combustion?
   (a) Burning of LPG
   (b) Bursting of a firecracker
   (c) Burning of white phosphorus at room temperature
   (d) Burning of a matchstick

6. Water cannot be used to extinguish fires caused by:
   (a) Wood
   (b) Paper
   (c) Electrical equipment
   (d) Cloth

7. The calorific value of a fuel is expressed in:
   (a) Kilograms per kilojoule (kg/kJ)
   (b) Kilojoules per kilogram (kJ/kg)
   (c) Joules per second (J/s)
   (d) Kilocalories per gram (kcal/g)

8. Incomplete combustion of fuels like petrol or diesel produces which poisonous gas?
   (a) Carbon dioxide
   (b) Sulfur dioxide
   (c) Carbon monoxide
   (d) Nitrogen dioxide

9. Which phenomenon is caused by the excessive release of carbon dioxide from burning fossil fuels?
   (a) Acid rain
   (b) Ozone layer depletion
   (c) Global warming
   (d) Smog formation

10. Which of the following fuels has the highest calorific value?
    (a) Wood
    (b) Coal
    (c) LPG
    (d) Hydrogen

Answer Key for MCQs:

1. (c)
2. (b)
3. (c)
4. (c)
5. (c)
6. (c)
7. (b)
8. (c)
9. (c)
10. (d)

Study these notes thoroughly. Understanding the concepts like the fire triangle, types of combustion, flame structure, and the environmental impact of fuels is very important for your exams. Good luck!