Question 7
Name the unit in which the calorific value of a fuel is expressed.
The calorific value of a fuel is expressed in kilojoule per kg (kJ/kg).
Key Concepts Covered
This question tests your understanding of the following concepts from the chapter Combustion and Flame: Question, Unit, Calorific, Value, Fuel, Expressed. These are fundamental topics in Science that students are expected to master as part of the CBSE Class 8 curriculum.
A thorough understanding of these concepts will help you answer similar questions confidently in your CBSE examinations. These topics are frequently tested in both objective and subjective sections of Science papers. We recommend revising the relevant section of your textbook alongside practising these solved examples to build a strong foundation.
How to Approach This Question
Read the question carefully and identify what is being asked. Break down complex questions into smaller parts. Use the terminology and concepts discussed in this chapter. Structure your answer logically — begin with a definition or key statement, then provide supporting details. Review your answer to ensure it addresses all parts of the question completely.
Key Points to Remember
- Read the question carefully and identify all parts before answering.
- Use the terminology specific to this subject and chapter.
- Review the textbook content for this chapter before attempting questions.
- Practice writing concise, well-structured answers within time limits.
Practice more questions from Combustion and Flame — Science, Class 8 CBSE
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.