Enhanced Theory with Definitions, Examples & Concepts
Combustion: A chemical process in which a substance reacts with oxygen to give off heat and light. It is an oxidation reaction that releases energy.
Fuel: A substance that undergoes combustion to produce energy. Examples: Wood, coal, petrol, LPG, natural gas.
Ignition Temperature: The lowest temperature at which a combustible substance catches fire and starts burning.
🔥 FIRE TRIANGLE 🔥
Three Essential Requirements for Combustion:
🔺 FUEL (Combustible substance)
🔺 OXYGEN (Supporter of combustion)
🔺 HEAT (To reach ignition temperature)
Remove any one component → Fire stops!
Types of Combustion
1. Rapid Combustion
Definition: Combustion in which a large amount of heat and light are produced in a short time.
Examples:
• Burning of LPG in gas stove
• Lighting a matchstick
• Burning of candle
• Kerosene lamp burning
Characteristics: Fast reaction, high heat production, visible flame.
2. Spontaneous Combustion
Definition: Combustion that occurs automatically at room temperature without any external heating.
Examples:
• White phosphorus catches fire at 35°C
• Coal dust in mines
• Hay stacks in farms
• Oily rags in shops
⚠️ Safety: White phosphorus stored under water to prevent contact with air.
3. Explosion
Definition: Combustion reaction that produces a large amount of gas and heat energy in a fraction of second.
Examples:
• Bursting of crackers
• LPG cylinder blast
• Bomb explosions
• Coal mine explosions
⚠️ Danger: Sudden expansion of gases causes destruction and damage.
Flame Structure and Types
🔥 Structure of a Candle Flame 🔥
Three Distinct Zones:
Dark Zone (Innermost)
Luminous Zone (Middle)
Non-luminous Zone (Outermost)
| Zone |
Color |
Temperature |
Oxygen Supply |
Combustion Type |
| Dark Zone |
Black/Dark |
Lowest (~500°C) |
No oxygen |
No combustion (unburnt vapors) |
| Luminous Zone |
Bright Yellow |
Medium (~1000°C) |
Insufficient oxygen |
Incomplete combustion |
| Non-luminous Zone |
Blue/Colorless |
Highest (~1500°C) |
Sufficient oxygen |
Complete combustion |
Luminous Flame
Characteristics: Bright yellow color, produces light, less hot, incomplete combustion.
Examples: Candle flame, kerosene lamp, wood fire.
Cause: Insufficient oxygen supply leads to incomplete burning.
Non-luminous Flame
Characteristics: Blue color, less light, very hot, complete combustion.
Examples: Gas stove flame, Bunsen burner, LPG flame.
Cause: Sufficient oxygen supply ensures complete burning.
Combustible and Non-combustible Substances
| Combustible Substances |
Non-combustible Substances |
| Wood, Paper, Cloth |
Stone, Glass, Iron |
| Petrol, Diesel, Kerosene |
Water, Sand, Cement |
| LPG, CNG, Natural Gas |
Asbestos, Concrete |
| Coal, Charcoal |
Carbon dioxide, Nitrogen |
| Alcohol, Cooking Oil |
Salt, Sugar (in solid form) |
Ignition Temperatures: White Phosphorus (35°C) | Paper (230°C) | Wood (260°C) | Kerosene (38°C) | Coal (500°C)
Fuel Types and Calorific Values
Calorific Value: The amount of heat energy produced on complete combustion of 1 kg of a fuel. Unit: kJ/kg or kcal/kg.
| Fuel Type |
Examples |
Calorific Value (kJ/kg) |
Advantages |
Disadvantages |
| Solid Fuels |
Wood, Coal, Charcoal |
17,000 - 30,000 |
Easy to store, transport |
Ash production, pollution |
| Liquid Fuels |
Petrol, Diesel, Kerosene |
45,000 - 50,000 |
High calorific value, clean burning |
Expensive, fire hazard |
| Gaseous Fuels |
LPG, CNG, Natural Gas |
50,000 - 55,000 |
Cleanest burning, high efficiency |
Storage problems, leakage risk |
Fire Control and Extinguishers
Principle of Fire Control: Remove one or more components of the fire triangle - fuel, oxygen, or heat.
Water Fire Extinguisher
How it works:
• Cools the burning material below ignition temperature
• Produces steam that displaces oxygen
• Suitable for: Wood, paper, cloth fires
⚠️ Never use on: Electrical fires, oil fires
CO₂ Fire Extinguisher
How it works:
• CO₂ is heavier than air
• Forms a blanket around fire
• Cuts off oxygen supply
• Suitable for: Electrical fires, oil fires
Advantage: Leaves no residue, safe for equipment.
Foam Extinguisher
How it works:
• Forms a layer over burning liquid
• Prevents oxygen from reaching fuel
• Suitable for: Liquid fuel fires (petrol, oil)
Sand/Soil
How it works:
• Cuts off air supply
• Absorbs heat
• Suitable for: Small fires, liquid spills
Use: Emergency situations, outdoor fires.
Harmful Effects of Burning Fuels
⚠️ Air Pollution Effects:
• Carbon Monoxide (CO): Poisonous gas, causes suffocation
• Carbon Dioxide (CO₂): Greenhouse gas, global warming
• Sulfur Dioxide (SO₂): Acid rain, respiratory problems
• Nitrogen Oxides (NOₓ): Smog formation, lung damage
• Particulate Matter: Breathing difficulties, lung diseases
Acid Rain Formation
Step 1: Burning of coal/petroleum → Release of SO₂ and NOₓ
Step 2: SO₂ + H₂O → H₂SO₃ (Sulfurous acid)
Step 3: 2SO₂ + O₂ → 2SO₃ (Sulfur trioxide)
Step 4: SO₃ + H₂O → H₂SO₄ (Sulfuric acid)
Result: Acidic rain (pH < 5.6) → Damages buildings, plants, aquatic life
Global Warming: Increased CO₂ levels trap heat in atmosphere → Rising temperatures → Climate change → Melting ice caps → Sea level rise.
Ideal Fuel Characteristics
Properties of an Ideal Fuel:
• High calorific value (more heat per unit mass)
• Easy availability and low cost
• Easy to store and transport
• Safe to handle and use
• Burns completely without leaving residue
• Does not produce harmful gases
• Burns at moderate rate (not too fast/slow)
• Low ignition temperature for easy lighting
Reality Check: No fuel is perfectly ideal. LPG and CNG come closest to ideal fuel properties for domestic use.
Section B: Short Answer Questions
Q1. What is combustion? Name the three requirements for combustion.
Answer: Combustion is a chemical process where a substance reacts with oxygen to give off heat and light. Three requirements: Fuel, Oxygen, and Heat (ignition temperature).
Q2. Define ignition temperature with examples.
Answer: Ignition temperature is the lowest temperature at which a combustible substance catches fire. Examples: White phosphorus (35°C), Paper (230°C), Wood (260°C).
Q3. Distinguish between rapid combustion and spontaneous combustion.
Answer: Rapid combustion produces heat and light quickly with external heating (LPG burning). Spontaneous combustion occurs automatically at room temperature without external heating (white phosphorus).
Q4. Why is white phosphorus stored under water?
Answer: White phosphorus has very low ignition temperature (35°C) and catches fire in air at room temperature. Water prevents its contact with oxygen in air.
Q5. Name the three zones of a candle flame and their characteristics.
Answer: Dark zone (innermost, no combustion, coolest), Luminous zone (middle, incomplete combustion, yellow), Non-luminous zone (outermost, complete combustion, hottest, blue).
Q6. What is calorific value? Give units.
Answer: Calorific value is the amount of heat energy produced on complete combustion of 1 kg of fuel. Units: kJ/kg or kcal/kg.
Q7. How does CO₂ extinguish fire?
Answer: CO₂ is heavier than air and forms a blanket around fire, cutting off oxygen supply. It also doesn't support combustion.
Q8. Why should we not use water on electrical fires?
Answer: Water conducts electricity and can cause electric shock. It may also damage electrical equipment. Use CO₂ extinguisher instead.
Q9. What is acid rain? How is it formed?
Answer: Acid rain is rain with pH less than 5.6. It forms when SO₂ and NOₓ from burning fuels react with water vapor to form sulfuric and nitric acids.
Q10. List four characteristics of an ideal fuel.
Answer: High calorific value, easy availability, safe handling, burns completely without harmful gases and residue.
Section C: Solutions (Minimum Words)
Q1. Explain the fire triangle and its importance in fire control.
Solution: Fire triangle shows three requirements for combustion: fuel, oxygen, and heat. Removing any one component stops fire. Fire control methods: water removes heat, CO₂ cuts oxygen, sand/foam blocks air supply. Understanding helps choose appropriate extinguisher for different fire types.
Q2. Describe the structure of a candle flame with diagram explanation.
Solution: Candle flame has three zones: 1) Dark zone (innermost): unburnt wax vapors, no oxygen, coolest (~500°C). 2) Luminous zone (middle): incomplete combustion, insufficient oxygen, yellow color, medium temperature (~1000°C). 3) Non-luminous zone (outermost): complete combustion, sufficient oxygen, blue/colorless, hottest (~1500°C).
Q3. Compare different types of combustion with examples.
Solution: Rapid combustion: fast burning with heat/light (LPG, matchstick). Spontaneous combustion: automatic burning at room temperature (white phosphorus at 35°C, coal dust). Explosion: instant combustion producing gases and energy (crackers, bomb). Each type differs in speed, heat production, and oxygen requirement.
Q4. Explain different types of fire extinguishers and their uses.
Solution: Water: cools burning material, produces steam (for wood, paper). CO₂: heavier than air, cuts oxygen supply (electrical, oil fires). Foam: forms layer over liquid fuel, prevents oxygen contact (petrol fires). Sand: absorbs heat, blocks air supply (emergency use). Choose based on fire type for safety.
Q5. Discuss harmful effects of burning fossil fuels on environment.
Solution: Burning fuels produces: CO (poisonous, suffocation), CO₂ (global warming, greenhouse effect), SO₂ (acid rain, respiratory problems), NOₓ (smog, lung damage), particulate matter (breathing difficulties). Leads to climate change, building damage, health issues, ecosystem disruption.
Q6. Compare solid, liquid, and gaseous fuels with advantages and disadvantages.
Solution: Solid fuels (wood, coal): easy storage/transport but produce ash, pollution. Liquid fuels (petrol, diesel): high calorific value, clean burning but expensive, fire hazard. Gaseous fuels (LPG, CNG): cleanest burning, highest efficiency but storage problems, leakage risk. Calorific values: gaseous > liquid > solid.
Q7. Explain the formation and effects of acid rain.
Solution: Formation: Burning coal/petroleum releases SO₂, NOₓ → react with water vapor → form H₂SO₄, HNO₃ → acidic rain (pH < 5.6). Effects: damages buildings (especially marble), kills aquatic life, harms plants, corrodes metals, reduces soil fertility, affects human health.
Q8. What makes a fuel ideal? Evaluate LPG as a fuel.
Solution: Ideal fuel characteristics: high calorific value, easy availability, safe handling, complete combustion, no harmful products. LPG evaluation: high calorific value (50,000 kJ/kg), burns completely, produces less pollution, easy to control, but expensive and requires special storage. Nearly ideal for domestic use.
Q9. Distinguish between luminous and non-luminous flames with examples.
Solution: Luminous flame: bright yellow, produces light, incomplete combustion due to insufficient oxygen, less hot, produces carbon particles (candle, kerosene lamp). Non-luminous flame: blue/colorless, complete combustion with sufficient oxygen, very hot, clean burning (gas stove, Bunsen burner). Used for different purposes based on requirements.
Q10. Explain global warming and its relation to combustion of fuels.
Solution: Global warming: increase in Earth's average temperature due to greenhouse gases. Combustion produces CO₂ which traps heat in atmosphere → enhanced greenhouse effect → rising temperatures → climate change → melting ice caps → sea level rise → extreme weather patterns. Reducing fossil fuel use essential for climate protection.