Question 10
Which zone of a flame does a goldsmith use for melting gold and silver and why?
Goldsmith uses the outermost zone of a flame for melting gold and silver because it is the hottest zone of the flame undergoing complete combustion. Hence, it easily melts gold and silver metals.
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.