Chapter 6: Combustion and Flame
दहन और ज्वाला (ज्वलन ताप, ज्वाला के क्षेत्र, ज्वलनशीलता, कैलोरी मान एवं प्रदूषण)
Don't Let a 20-Minute Bubbling Delay Cost You CTET Qualification
In offline CTET exams, bubbling 150 bubbles takes 18 to 22 minutes of mechanical time. A single misaligned row can trigger an 8-question domino error. Practice bubble-filling this chapter's solved MCQs inside Mentora's official OMR simulator.
- Combustion: Chemical process in which a substance reacts with oxygen to give off heat and light. Essential requirements: Fuel + Oxygen ($O_2$) + Heat to reach Ignition Temperature.
- Ignition Temperature (ज्वलन ताप): The lowest temperature at which a combustible substance catches fire. A substance cannot catch fire as long as its temperature remains below its ignition temperature (e.g. paper cup with water does not burn over candle flame because heat transfers to water via conduction).
- Inflammable Substances: Materials with very low ignition temperatures that catch fire easily with a spark (Petrol, Alcohol, LPG).
- Fire Fighting Mechanism: Water cools fire below ignition temperature; however, WATER MUST NEVER BE USED FOR ELECTRICAL FIRES (risk of electrocution) OR OIL/PETROL FIRES (oil floats on water and spreads fire). $CO_2$ is the best extinguisher: heavier than $O_2$, blankets fuel cut off oxygen, and expands with intense cooling.
- Flame Anatomy Zones (From Inside to Outside):
- 1. Innermost Dark Zone: Contains unburnt combustible wax vapours; black; lowest temperature (least hot).
- 2. Middle Luminous Zone: Incomplete combustion occurs; yellow color; moderately hot; emits glowing carbon soot particles.
- 3. Outermost Non-Luminous Zone: Complete combustion occurs; faint blue color; HOTTEST ZONE of the flame. Goldsmiths blow this outermost blue zone with a metallic blowpipe to melt gold and silver quickly.
- Calorific Value (ऊष्मीय मान): Amount of heat energy released upon complete combustion of $1\text{ kg}$ of fuel, expressed in $\text{kJ/kg}$.
- - Hydrogen has HIGHEST calorific value: $150,000\text{ kJ/kg}$ (used in rockets; dangerous for domestic storage).
- - LPG: $55,000\text{ kJ/kg}$; Methane/CNG: $50,000\text{ kJ/kg}$; Petrol/Diesel: $45,000\text{ kJ/kg}$; Wood: $17,000-22,000\text{ kJ/kg}$; Cow dung cake: $6,000-8,000\text{ kJ/kg}$.
- Pollution Products: Incomplete combustion yields poisonous Carbon Monoxide ($CO$), which binds irreversibly with hemoglobin. Unburnt carbon particles cause respiratory ailments (asthma). Oxides of sulfur ($SO_2$) and nitrogen ($NO_2$) dissolve in raindrops forming Acid Rain.
- Trap: Which zone of the candle flame is the hottest? Correct: The OUTERMOST BLUE ZONE (complete combustion), used by goldsmiths.
- Trap: Which fuel has the highest calorific value among Hydrogen, LPG, Petrol, and Wood? Correct: HYDROGEN ($150,000\text{ kJ/kg}$).
- Trap: Why does a paper cup containing water not burn when held over a flame? Correct: Heat absorbed by the paper is transferred to water by CONDUCTION, keeping paper temperature below its ignition point.
NCERT Core Facts: Flame Zones, Heat Values & Combustion Dynamics
Candle flame zones and calorific value calculations are core staples of CTET Paper 2:
1. Cross-Sectional Architecture of a Flame
▲ [Outermost Zone] ── Non-luminous blue; Complete combustion; HOTTEST (Goldsmiths)
╱ ╲
╱ ╲
│ ┌───┐ │ [Middle Zone] ── Luminous yellow; Incomplete combustion; Moderately hot
│ │ │ │
│ │ ■ │ │ [Innermost Zone] ── Dark black; Unburnt wax vapor; LEAST HOT2. High-Yield Calorific Value Comparison ( ext{kJ/kg})
| Fuel | Calorific Value ( ext{kJ/kg}) | Combustion State | CTET MCQ Focus |
|---|---|---|---|
| Cow dung cake | 6,000 - 8,000 | Solid, smoky | Lowest efficiency; high ash residue |
| Wood | 17,000 - 22,000 | Solid | Causes deforestation and particulate smoke |
| Coal | 25,000 - 33,000 | Solid | Releases SO_2 triggering acid rain |
| Petrol / Diesel | 45,000 | Liquid | High energy density transport fuels |
| LPG | $55,000$ | Gas | Clean domestic fuel; zero smoke |
| Hydrogen Gas | $150,000$ | Gas | Highest calorific value known; rocket propellant |
Test Your Mastery: Combustion and Flame
Authentic CBSE exam questions tested directly from this NCERT chapter.
A goldsmith blows through a metallic blowpipe to direct a specific zone of a candle flame onto small pieces of gold. Which zone does he use, and why?
एक सुनार सोने के छोटे टुकड़ों को पिघलाने के लिए धातु की फूँकनी से मोमबत्ती की ज्वाला के एक विशिष्ट क्षेत्र को फूंकता है। वह किस क्षेत्र का उपयोग करता है और क्यों?
Arrange the following fuels in INCREASING order of their calorific values: Wood, Hydrogen, Cow dung cake, LPG.
निम्नलिखित ईंधनों को उनके कैलोरी मान (ऊष्मीय मान) के बढ़ते क्रम में व्यवस्थित करें: लकड़ी, हाइड्रोजन, गोबर के उपले, एलपीजी।
Why do goldsmiths use the outermost blue zone of a flame for melting precious metals?
Because the outermost non-luminous blue zone is the hottest part of the flame, where complete combustion takes place with excess oxygen.
Why should water never be thrown on an electrical or petrol fire?
Water conducts electricity, creating lethal shock hazard. On petrol fires, water is denser than oil, causing petrol to float on top and spread the blaze.