Question 9
It is difficult to burn a heap of green leaves but dry leaves catch fire easily. Explain.
A heap of green leaves contains a lot of moisture due to which its ignition temperature is high. Hence, it does not catch fire easily.
On the other hand there is no or very less moisture content in dry leaves, hence their ignition temperature is low and they catch fire easily.
Chapter Overview: Combustion and Flame
This chapter explains how substances burn (combustion), the three types of combustion, the structure of a flame with its three zones, and the concept of calorific value. Fire safety and the environmental effects of burning fossil fuels are practical, exam-relevant topics.
Board Exam Weightage: 4-6 marks | Difficulty: Moderate
Key Formulas & Concepts
| Concept | Details |
|---|---|
| Calorific Value | Heat energy produced by complete combustion of 1 kg of fuel (kJ/kg) |
| Ignition Temperature | Minimum temperature at which a substance catches fire |
| Fire Triangle | Three requirements: fuel + oxygen + ignition temperature |
| Ideal Fuel Criteria | High calorific value, low residue, moderate ignition temperature, low cost, low pollution |
Must-Know Concepts
- Three conditions for combustion: combustible substance + oxygen + heat (ignition temperature)
- Rapid combustion requires external ignition; spontaneous combustion is self-starting; explosion is very rapid
- Candle flame zones: dark (coldest, unburnt) → luminous (middle, yellow) → non-luminous (hottest, blue)
- Water cannot be used on oil fires (oil floats) or electrical fires (electrocution risk)
- Hydrogen has the highest calorific value (1,50,000 kJ/kg)
- Burning fossil fuels produces CO2 (global warming), CO (poisoning), SO2 (acid rain)
Types of Combustion
| Type | Speed | Ignition | Examples |
|---|---|---|---|
| Rapid | Quick | External source needed | LPG burning, candle, matchstick |
| Spontaneous | Variable | Self-igniting (no external heat) | Forest fires, white phosphorus, coal mines |
| Explosion | Very rapid | Sudden gas expansion | Firecrackers, dynamite, bombs |
Common Mistakes to Avoid
- Thinking all burning produces a flame (charcoal glows without flame)
- Confusing the three zones of a candle flame
- Believing water can extinguish all types of fires
- Not understanding that calorific value is per kg of fuel
Scoring Tips
- Memorise calorific values of 5 common fuels for comparison questions
- Draw and label the candle flame with all three zones
- Learn fire safety rules as practical life skills
- Connect CO2 emissions to global warming for HOTS questions
Frequently Asked Questions
Q: How many marks does this chapter carry in the exam?
A: Approximately 4-6 marks in the annual exam.
Q: What question types are commonly asked?
A: MCQs (1 mark), Very Short Answer (2 marks), Short Answer (3 marks), and Long Answer / Diagram (5 marks).
Q: Is this chapter important for competitive exams?
A: Yes, concepts from this chapter appear in NTSE, Olympiad, and other science competitions.