Question 5
Give reasons.
(a) Water is not used to control fires involving electrical equipment.
(b) LPG is a better domestic fuel than wood.
(c) Paper by itself catches fire easily whereas a piece of paper wrapped around an aluminium pipe does not.
(a) If electrical equipment is on fire, water being a good conductor, may conduct electricity and harm those trying to douse the fire. Hence, water is not a good alternative to stop such fires. Instead, carbon dioxide (CO2) is the best extinguisher of fires involving electrical equipment.
(b) LPG is a better domestic fuel than wood because :
- LPG doesn't produce smoke and causes less pollution. Wood on the other hand produces lots of smoke and causes more pollution.
- No cutting of trees is required to produce LPG whereas wood is obtained by cutting trees which leads to deforestation.
- LPG has a higher calorific value of 55000 kJ/kg whereas wood has a lower calorific value ranging between 17000 and 22000 kJ/kg.
- LPG is easier to store and transport whereas wood requires a lot of storage space and transport is difficult due to more weight.
- Ignition temperature of LPG is low and that of wood is high.
(c) Paper by itself catches fire easily because it has low ignition temperature. When a piece of paper is wrapped around an aluminium pipe and heated, its ignition temperature is not attained so soon because the heat supplied is absorbed by the aluminium pipe. Hence, it does not 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.