Class 8 CBSE Combustion And Flames 🔥 Notes by Abhinav Sir

🔥 Combustion and Flame 🔥

Complete CBSE Class 8 Science Notes with 80 Questions

📋 Table of Contents

• 1. What is Combustion?
• 2. Conditions for Combustion
• 3. Types of Combustion
• 4. Structure of Flame
• 5. Fuels and Classification
• 6. Fire Control Methods
• 7. Environmental Effects
• 8.History of Matchstick
• 9. 80 Practice Questions

1. What is Combustion? 🔥

Definition: Combustion is a chemical process in which a substance reacts with oxygen to produce heat and light.

Key Points:

• Also known as burning
• Produces heat and light energy
• Requires oxygen from air
• Chemical change - irreversible process

Examples of Combustion:

• Burning wood → Heat + Light + CO₂ + Water vapor
• Magnesium burning → MgO + Heat + Light
• LPG combustion → CO₂ + H₂O + Heat + Light

2. Essential Conditions for Combustion (Fire Triangle) 🔺

Fire Triangle - Three Essential Conditions:

1. Fuel (Combustible Substance) - Material that can burn
2. Oxygen (Supporter of Combustion) - Usually from air
3. Heat (Ignition Temperature) - Minimum temperature needed

If ANY one condition is missing, combustion cannot occur.

Important Definitions:

Ignition Temperature: The lowest temperature at which a substance catches fire and starts burning.

Inflammable Substances: Materials with very low ignition temperature that catch fire easily.
Examples: Petrol, LPG, alcohol, kerosene

Combustible Substances: Materials that can burn in air.
Examples: Wood, paper, coal, wax

Non-combustible Substances: Materials that do not burn in air.
Examples: Water, sand, stone, glass, iron

3. Types of Combustion ⚡

3.1 Rapid Combustion

• Fast burning with heat and light
• External heat required initially
• Examples: LPG in gas stove, matchstick burning

3.2 Spontaneous Combustion

• Self-ignition without external heat
• Occurs due to internal chemical reactions
• Examples: White phosphorus at room temperature

3.3 Explosion

• Sudden reaction with heat, light, and sound
• Large amount of gas released quickly
• Examples: Firecrackers, bombs

4. Structure of Flame 🕯️

Flame Definition: The visible gaseous part of fire formed by substances that vaporize during burning.

Three Zones of Candle Flame:

Inner Zone
Dark/Black
Coolest
Unburnt vapors

Middle Zone
Yellow/Orange
Moderate heat
Partial combustion

Outer Zone
Blue
Hottest
Complete combustion

Practical Application: Goldsmiths use the outermost (blue) zone for melting gold and silver because it's the hottest part.

5. Fuels and Their Classification ⛽

Fuel: Any substance that burns in air to produce heat and light energy.

Classification of Fuels:

Type	Examples	Advantages	Disadvantages
Solid Fuels	Coal, Wood, Charcoal	Easy to store	Produce smoke
Liquid Fuels	Kerosene, Petrol, Diesel	High calorific value	Inflammable
Gaseous Fuels	LPG, CNG, Natural gas	Clean burning	Special storage needed

Calorific Value

Calorific Value: The amount of heat energy produced by complete combustion of 1 kg of fuel.
Unit: Kilojoule per kilogram (kJ/kg)

Fuel	Calorific Value (kJ/kg)	State
Hydrogen	150,000	Gas
LPG	55,000	Gas
CNG/Methane	50,000	Gas
Kerosene/Petrol/Diesel	45,000	Liquid
Coal	25,000-33,000	Solid
Wood	17,000-22,000	Solid

6. Fire Control and Extinguishers 🧯

Methods to Control Fire:

1. Remove fuel source - Cut off supply of combustible material
2. Cut off oxygen supply - Use blanket or CO₂
3. Lower temperature - Use water to cool below ignition point

Types of Fire Extinguishers:

1. Water Extinguisher

• Best for: Wood, paper, cloth fires
• Never use for: Oil fires or electrical equipment

2. Carbon Dioxide (CO₂) Extinguisher

• Best for: Oil, petrol, electrical fires
• How it works: CO₂ is heavier than air, cuts oxygen supply

7. Environmental Effects of Burning Fuels 🌍

Harmful Effects:

Air Pollution

• Unburnt carbon particles cause respiratory problems
• Sulfur dioxide causes breathing difficulties
• Particulate matter in air

Global Warming

• Increased CO₂ levels in atmosphere
• Greenhouse effect
• Climate change

CNG - A Cleaner Fuel

Why CNG is Better:

• Burns completely with less smoke
• Produces less carbon monoxide
• Higher calorific value
• Reduces air pollution in cities

8. The History of the Matchstick 📜

From Ancient Times to Modern Safety: The journey of the matchstick is a fascinating story of chemical discovery and safety innovation over thousands of years.

Key Historical Milestones:

• Ancient Origins (~5,000 years ago): The earliest known predecessors were small pinewood sticks dipped in sulfur, used by the ancient Egyptians. These needed an external flame to light and were not true matches.
• The First Friction Match (~200 years ago): The modern match began with the discovery of phosphorus. The first friction matches used a dangerous mix of antimony trisulfide, potassium chlorate, and white phosphorus on the match head.
• The Danger of White Phosphorus: White phosphorus is highly volatile and poisonous. It would often ignite accidentally from friction or heat and caused serious health problems for match factory workers.
• The Invention of the Safety Match: The breakthrough came with the use of red phosphorus, which is much less volatile and not poisonous. This made matches significantly safer.

How a Modern Safety Match Works:

1. Match Head: Contains antimony trisulfide and potassium chlorate.
2. Striking Surface (on the box): Contains powdered glass (for friction) and a small amount of red phosphorus.
3. The Science: When the match is struck, friction generates heat. This heat converts a tiny amount of red phosphorus into white phosphorus, which immediately ignites and starts the combustion of the chemicals on the match head.

Key Safety Innovation: By separating the chemicals (red phosphorus on the box and other chemicals on the match head), the safety match ensures that it can only be lit on its specific striking surface.

8. 80 Practice Questions with Detailed Explanations 📝

A. Very Short Answer Questions (1 Mark Each) - 25 Questions

Q1. What is combustion?

Answer: Combustion is a chemical process in which a substance reacts with oxygen to produce heat and light.

Q2. What is the unit of calorific value?

Answer: Kilojoule per kilogram (kJ/kg).

Q3. Which gas is produced during incomplete combustion?

Answer: Carbon monoxide (CO).

Q4. What is ignition temperature?

Answer: The lowest temperature at which a substance catches fire and starts burning.

Q5. Name two inflammable substances.

Answer: Petrol and LPG.

Q6. Which zone of flame is hottest?

Answer: Outermost zone (non-luminous/blue zone).

Q7. What is the color of the middle zone of flame?

Answer: Yellow or orange.

Q8. Name a solid fuel.

Answer: Coal (or wood, charcoal).

Q9. Which gas is used in fire extinguishers?

Answer: Carbon dioxide (CO₂).

Q10. What causes acid rain?

Answer: Sulfur dioxide and nitrogen oxides dissolving in rainwater.

Q11. What is a fuel?

Answer: A substance that burns in air to produce heat and light.

Q12. Which fuel has the highest calorific value?

Answer: Hydrogen (150,000 kJ/kg).

Q13. What is the color of LPG flame?

Answer: Blue.

Q14. Name a liquid fuel used in homes.

Answer: Kerosene.

Q15. What type of combustion occurs in crackers?

Answer: Explosion.

Q16. Why is white phosphorus stored under water?

Answer: To prevent spontaneous combustion at room temperature.

Q17. Which zone of flame has unburnt fuel?

Answer: Inner zone (dark zone).

Q18. What is produced when magnesium burns?

Answer: Magnesium oxide (MgO) with heat and light.

Q19. Name the supporter of combustion.

Answer: Oxygen.

Q20. What is the full form of CNG?

Answer: Compressed Natural Gas.

Q21. Which gas is responsible for global warming?

Answer: Carbon dioxide (CO₂).

Q22. Name a non-combustible substance.

Answer: Water (or sand, stone, glass).

Q23. Which extinguisher is used for electrical fires?

Answer: CO₂ (Carbon dioxide) extinguisher.

Q24. What happens when hydrogen burns?

Answer: Water vapor (H₂O) and heat are produced.

Q25. Which flame gives more heat - luminous or non-luminous?

Answer: Non-luminous flame (blue flame) gives more heat.

B. Short Answer Questions (2-3 Marks Each) - 20 Questions

Q26. List three conditions necessary for combustion.

Answer: The three essential conditions for combustion are:

1. Presence of fuel - A combustible substance
2. Supply of oxygen - Usually from air
3. Attainment of ignition temperature - Minimum temperature for burning

These three conditions form the "Fire Triangle."

Q27. Differentiate between combustible and non-combustible substances.

Answer:

Combustible substances: Can burn in air to produce heat and light. Examples: Wood, paper, coal

Non-combustible substances: Cannot burn in air under normal conditions. Examples: Water, sand, stone

Q28. Why is CO₂ used as fire extinguisher?

Answer: CO₂ is used as fire extinguisher because:

• It is heavier than air and settles over fire
• Cuts off oxygen supply by forming a blanket
• Does not burn itself
• Leaves no residue

Q29. What are the harmful effects of burning fossil fuels?

Answer: Harmful effects include:

• Air pollution from unburnt particles
• Carbon monoxide production (poisonous gas)
• Global warming due to increased CO₂
• Acid rain from sulfur and nitrogen oxides
• Respiratory problems and health issues

Q30. Explain the structure of candle flame.

Answer: A candle flame has three zones:

1. Inner Zone: Dark, coolest, contains unburnt wax vapors
2. Middle Zone: Yellow, moderate heat, partial combustion
3. Outer Zone: Blue, hottest, complete combustion

Q31. How do we control fire?

Answer: Fire can be controlled by:

1. Removing fuel source
2. Cutting off oxygen supply
3. Lowering temperature below ignition point

Methods: Water, CO₂, foam, sand, or blankets.

Q32. What is calorific value? Give examples.

Answer:

Definition: Amount of heat produced by complete burning of 1 kg fuel

Unit: kJ/kg

Examples: Hydrogen (150,000), LPG (55,000), Coal (30,000)

Q33. Why is CNG better than petrol?

Answer: CNG is better because:

• Burns more cleanly with less smoke
• Produces less pollution
• Higher calorific value
• More economical
• Reduces air pollution in cities

Q34. What happens when you blow air into fire?

Answer: When air is blown into fire:

• Increased oxygen supply
• Fire burns brighter and hotter
• Rate of combustion increases
• Complete combustion occurs

Q35. Distinguish between rapid and spontaneous combustion.

Answer:

Rapid Combustion: Fast burning with external heat. Example: LPG

Spontaneous Combustion: Self-ignition without external heat. Example: White phosphorus

Q36. Why is water not used for oil fires?

Answer: Water is not used for oil fires because:

• Oil floats on water (lighter density)
• Fire continues burning on oil surface
• Water may spread the burning oil
• Can cause dangerous splashing

Q37. What are the characteristics of good fuel?

Answer: Good fuel should have:

• High calorific value
• Easy ignition
• Clean burning (no smoke)
• No harmful residue
• Easy storage and transport
• Economical

Q38. How does forest fire occur?

Answer: Forest fires occur due to:

• High temperature in summer
• Dry vegetation reaches ignition temperature
• Spontaneous combustion of dry leaves
• Lightning strikes
• Human activities (cigarettes, campfires)

Q39. Explain why matchstick doesn't catch fire on its own.

Answer: Matchstick doesn't catch fire because:

• Room temperature is below ignition temperature
• External heat (friction) is needed
• Striking provides necessary heat energy
• Only then chemical reaction starts

Q40. What is the role of oxygen in combustion?

Answer: Oxygen's role in combustion:

• Acts as supporter of combustion
• Combines with fuel to produce energy
• More oxygen = better burning
• No oxygen = no combustion
• Essential part of fire triangle

Q41. How is LPG better than wood as fuel?

Answer: LPG is better than wood because:

• Higher calorific value (55,000 vs 20,000 kJ/kg)
• Burns without smoke
• No ash production
• Easy to control
• Instant ignition
• Clean and hygienic

Q42. What precautions should be taken while using LPG?

Answer: LPG safety precautions:

• Check for gas leaks regularly
• Turn off gas when not in use
• Ensure proper ventilation
• Keep away from heat sources
• Don't smoke near gas cylinder
• Regular servicing of equipment

Q43. Why does a candle go out when covered with glass?

Answer: Candle goes out because:

• Limited oxygen inside glass
• Oxygen gets consumed in burning
• No fresh air can enter
• Without oxygen, combustion stops
• Demonstrates importance of oxygen for fire

Q44. How do different fire extinguishers work?

Answer: Fire extinguishers work by:

• Water: Cools and produces steam
• CO₂: Cuts oxygen supply
• Foam: Creates barrier on liquid surface
• Dry powder: Releases CO₂ when heated

Q45. Why is complete combustion better than incomplete combustion?

Answer: Complete combustion is better because:

• Produces more heat energy
• No poisonous CO gas
• Less pollution
• No unburnt particles
• More efficient fuel usage
• Cleaner environment

C. Multiple Choice Questions - 15 Questions

Q46. Which of the following is essential for combustion?

• (a) Fuel only
• (b) Oxygen only
• (c) Heat only
• (d) All three: fuel, oxygen, and heat

Answer: (d) All three components of fire triangle are essential.

Q47. The calorific value of LPG is:

• (a) 45,000 kJ/kg
• (b) 55,000 kJ/kg
• (c) 50,000 kJ/kg
• (d) 25,000 kJ/kg

Answer: (b) LPG has calorific value of 55,000 kJ/kg.

Q48. Fire produced by oil should be extinguished by:

• (a) Water
• (b) CO₂
• (c) Sand
• (d) Both (b) and (c)

Answer: (d) Both CO₂ and sand work for oil fires, never water.

Q49. Which zone of candle flame is used by goldsmiths?

• (a) Inner zone
• (b) Middle zone
• (c) Outer zone
• (d) All zones

Answer: (c) Outer zone is hottest for melting metals.

Q50. Spontaneous combustion is seen in:

• (a) White phosphorus
• (b) Paper
• (c) Wood
• (d) Coal

Answer: (a) White phosphorus ignites at room temperature.

Q51. The gas produced during incomplete combustion is:

• (a) CO₂
• (b) CO
• (c) H₂O
• (d) O₂

Answer: (b) Carbon monoxide is produced in incomplete combustion.

Q52. CNG stands for:

• (a) Carbon Natural Gas
• (b) Compressed Natural Gas
• (c) Combined Natural Gas
• (d) Combustible Natural Gas

Answer: (b) CNG is Compressed Natural Gas.

Q53. Which fuel has the highest calorific value?

• (a) Coal
• (b) LPG
• (c) Hydrogen
• (d) Petrol

Answer: (c) Hydrogen has highest calorific value (150,000 kJ/kg).

Q54. The color of the middle zone of candle flame is:

• (a) Blue
• (b) Yellow
• (c) Black
• (d) White

Answer: (b) Middle zone is yellow due to partial combustion.

Q55. Acid rain is caused by:

• (a) CO₂
• (b) H₂O
• (c) SO₂ and NO₂
• (d) O₂

Answer: (c) Sulfur and nitrogen oxides cause acid rain.

Q56. Which extinguisher should NOT be used on electrical fires?

• (a) Water
• (b) CO₂
• (c) Dry powder
• (d) Foam

Answer: (a) Water conducts electricity, dangerous for electrical fires.

Q57. The main component of natural gas is:

• (a) Propane
• (b) Butane
• (c) Methane
• (d) Ethane

Answer: (c) Methane (CH₄) is the main component of natural gas.

Q58. Which type of combustion occurs in car engines?

• (a) Rapid combustion
• (b) Spontaneous combustion
• (c) Explosion
• (d) Slow combustion

Answer: (a) Car engines use controlled rapid combustion.

Q59. Which flame is preferred for cooking?

• (a) Yellow flame
• (b) Blue flame
• (c) Orange flame
• (d) Red flame

Answer: (b) Blue flame gives more heat and is cleaner.

Q60. Global warming is mainly caused by:

• (a) CO₂
• (b) CO
• (c) H₂O
• (d) N₂

Answer: (a) CO₂ is the main greenhouse gas causing global warming.

D. Long Answer Questions (5 Marks Each) - 10 Questions

Q61. Describe the different types of combustion with examples.

Answer:

1. Rapid Combustion:

• Fast burning with heat and light
• External heat needed to start
• Examples: LPG burning, candle flame
• Controlled and useful process

2. Spontaneous Combustion:

• Self-ignition without external heat
• Very low ignition temperature
• Examples: White phosphorus, forest fires
• Dangerous and unpredictable

3. Explosion:

• Very fast reaction with sound
• Large volume of gases produced
• Examples: Firecrackers, bombs
• Destructive but has specific uses

Q62. Explain the environmental effects of burning fossil fuels and suggest solutions.

Answer:

Environmental Effects:

• Air Pollution: Smoke, soot, unburnt particles
• Global Warming: CO₂ increases greenhouse effect
• Acid Rain: SO₂ and NO₂ form acids in rain
• Health Problems: Respiratory diseases, cancer
• Climate Change: Extreme weather patterns

Solutions:

• Use cleaner fuels like CNG, hydrogen
• Promote renewable energy sources
• Plant more trees for carbon absorption
• Improve vehicle fuel efficiency
• Use public transportation
• Develop electric vehicles

Q63. How do different fire extinguishers work? When should each be used?

Answer:

1. Water Extinguisher:

• Cools fuel below ignition temperature
• Steam displaces oxygen
• Use for: Wood, paper, cloth fires
• Never use for: Oil or electrical fires

2. CO₂ Extinguisher:

• Heavier than air, cuts oxygen
• Non-flammable and leaves no residue
• Use for: Oil, petrol, electrical fires
• Safe for delicate equipment

3. Foam Extinguisher:

• Creates barrier on liquid surface
• Prevents oxygen contact
• Use for: Liquid fuel fires

4. Dry Powder:

• Releases CO₂ when heated
• Multi-purpose extinguisher
• Use for: Various types of fires

Q64. Compare solid, liquid, and gaseous fuels with examples.

Answer:

Property	Solid Fuels	Liquid Fuels	Gaseous Fuels
Examples	Coal, Wood	Petrol, Kerosene	LPG, CNG
Storage	Easy	Moderate	Difficult
Pollution	High smoke	Moderate	Least
Efficiency	Low	High	Highest

Q65. Why is hydrogen considered the fuel of the future? What are its advantages and challenges?

Answer:

Why Hydrogen is Future Fuel:

• Highest calorific value (150,000 kJ/kg)
• Clean burning - produces only water
• Abundant in universe
• Renewable source

Advantages:

• Zero pollution
• No greenhouse gas emission
• High energy efficiency
• Can be produced from water

Challenges:

• High ignition temperature
• Expensive production
• Storage difficulties
• Safety concerns (highly inflammable)
• Technology still developing

Q66. Explain how CNG has helped reduce pollution in cities. Compare it with petrol.

Answer:

How CNG Reduces Pollution:

• Complete combustion produces less smoke
• Lower carbon monoxide emission
• Minimal particulate matter
• Reduced sulfur content
• Less contribution to smog formation

CNG vs Petrol Comparison:

• Emissions: CNG 20-30% less CO₂ than petrol
• Cost: CNG 40-60% cheaper
• Efficiency: CNG 50,000 kJ/kg vs Petrol 44,000 kJ/kg
• Environment: CNG much cleaner
• Health: Less respiratory problems with CNG

City Benefits:

• Improved air quality
• Reduced hospital visits for breathing problems
• Better visibility
• Slower climate change impact

Q67. Describe fire safety measures for home and school.

Answer:

Home Fire Safety:

• Install smoke detectors
• Keep fire extinguishers accessible
• Regular electrical wiring checks
• Safe storage of inflammable materials
• Plan escape routes
• Never leave candles unattended

School Fire Safety:

• Fire drill practices
• Emergency exit signs
• Fire extinguishers in each room
• First aid training
• Laboratory safety protocols
• No smoking policy

General Precautions:

• Know how to use fire extinguishers
• Never use water on electrical fires
• Call fire department immediately
• Stay low in smoke
• Don't go back into burning building

Q68. What is global warming? How does combustion contribute to it? Suggest prevention methods.

Answer:

Global Warming:

• Gradual increase in Earth's temperature
• Caused by greenhouse gases
• Main gas: Carbon dioxide (CO₂)
• Traps heat in atmosphere

How Combustion Contributes:

• Burning fossil fuels releases CO₂
• Deforestation for fuel reduces CO₂ absorption
• Industrial combustion increases emissions
• Vehicle combustion adds to pollution

Effects:

• Melting ice caps
• Rising sea levels
• Extreme weather
• Threats to biodiversity

Prevention Methods:

• Use renewable energy
• Plant more trees
• Improve fuel efficiency
• Use public transport
• Develop clean technologies

Q69. Why does a paper cup with water not catch fire when heated?

Answer:

Scientific Explanation:

• Water has high specific heat capacity
• Water absorbs heat from paper
• Paper temperature stays below ignition point
• Heat is used to warm water instead

Detailed Process:

• Heat applied to cup bottom
• Heat transfers to water through paper
• Water temperature rises slowly
• Paper remains cool due to water's cooling effect
• Even if water boils, steam takes away heat

Key Points:

• Water acts as heat sink
• Temperature regulation by water
• Paper ignition temperature not reached
• Demonstrates heat transfer principles

Without Water: Empty paper cup would immediately catch fire as there's nothing to absorb heat.

Q70. Explain the complete process of combustion with chemical equations.

Answer:

Process of Combustion:

1. Fuel is heated to ignition temperature
2. Fuel combines with oxygen from air
3. Chemical reaction produces heat and light
4. Products are formed (CO₂, H₂O, etc.)

Chemical Equations:

Complete Combustion:

• Methane: CH₄ + 2O₂ → CO₂ + 2H₂O + Heat
• Carbon: C + O₂ → CO₂ + Heat
• Hydrogen: 2H₂ + O₂ → 2H₂O + Heat
• Magnesium: 2Mg + O₂ → 2MgO + Heat + Light

Incomplete Combustion:

• Carbon: 2C + O₂ → 2CO + Heat
• Methane: 2CH₄ + 3O₂ → 2CO + 4H₂O + Heat

Energy Changes:

• Chemical energy → Heat + Light energy
• Exothermic reaction (heat released)
• Bond breaking requires energy
• Bond formation releases more energy

E. Numerical Problems - 10 Questions

Q71. If 2 kg of coal with calorific value 30,000 kJ/kg is burnt, how much heat energy is produced?

Answer:

Given: Mass = 2 kg, Calorific value = 30,000 kJ/kg

Formula: Heat energy = Mass × Calorific value

Calculation: Heat energy = 2 × 30,000 = 60,000 kJ

Answer: 60,000 kJ of heat energy is produced.

Q72. How much LPG is needed to produce 110,000 kJ of heat energy?

Answer:

Given: Heat needed = 110,000 kJ, LPG calorific value = 55,000 kJ/kg

Formula: Mass = Heat energy ÷ Calorific value

Calculation: Mass = 110,000 ÷ 55,000 = 2 kg

Answer: 2 kg of LPG is needed.

Q73. Compare the heat produced by 1 kg each of wood and hydrogen.

Answer:

Wood: 1 kg × 20,000 kJ/kg = 20,000 kJ

Hydrogen: 1 kg × 150,000 kJ/kg = 150,000 kJ

Difference: 150,000 - 20,000 = 130,000 kJ

Conclusion: Hydrogen produces 7.5 times more heat than wood.

Q74. If 5 kg of CNG produces 250,000 kJ of heat, what is its calorific value?

Answer:

Given: Mass = 5 kg, Heat produced = 250,000 kJ

Formula: Calorific value = Heat energy ÷ Mass

Calculation: Calorific value = 250,000 ÷ 5 = 50,000 kJ/kg

Answer: Calorific value of CNG is 50,000 kJ/kg.

Q75. Which is more economical: 10 kg coal at ₹5/kg or 2 kg LPG at ₹50/kg for same heat output?

Answer:

Coal: Heat = 10 × 30,000 = 300,000 kJ, Cost = 10 × 5 = ₹50

LPG: Heat = 2 × 55,000 = 110,000 kJ, Cost = 2 × 50 = ₹100

For same heat (110,000 kJ):

Coal needed = 110,000 ÷ 30,000 = 3.67 kg

Coal cost = 3.67 × 5 = ₹18.35

Answer: Coal is more economical (₹18.35 vs ₹100).

Q76. How much heat is required to burn 0.5 kg of kerosene completely?

Answer:

Given: Mass = 0.5 kg, Kerosene calorific value = 45,000 kJ/kg

Note: The question asks heat required, but combustion releases heat

Heat released: 0.5 × 45,000 = 22,500 kJ

Answer: 22,500 kJ of heat energy will be released.

Q77. A fuel has calorific value 40,000 kJ/kg. How much fuel is needed to produce same heat as 3 kg of petrol?

Answer:

Heat from petrol: 3 × 45,000 = 135,000 kJ

Fuel needed: 135,000 ÷ 40,000 = 3.375 kg

Answer: 3.375 kg of fuel is needed.

Q78. Calculate the efficiency if 2 kg of coal produces 50,000 kJ instead of expected heat.

Answer:

Expected heat: 2 × 30,000 = 60,000 kJ

Actual heat: 50,000 kJ

Efficiency = (Actual/Expected) × 100

Efficiency: (50,000/60,000) × 100 = 83.33%

Answer: Efficiency is 83.33%.

Q79. How many times more efficient is LPG than wood in terms of calorific value?

Answer:

LPG calorific value: 55,000 kJ/kg

Wood calorific value: 20,000 kJ/kg

Ratio: 55,000 ÷ 20,000 = 2.75

Answer: LPG is 2.75 times more efficient than wood.

Q80. If a stove has 60% efficiency, how much actual heat is obtained from 1 kg of LPG?

Answer:

Total heat available: 1 × 55,000 = 55,000 kJ

Efficiency: 60% = 0.6

Actual heat obtained: 55,000 × 0.6 = 33,000 kJ

Answer: 33,000 kJ of heat is actually obtained.

🎯 Complete Question Bank Summary

This comprehensive question bank contains exactly 80 questions covering:

• 25 Very Short Answer Questions (1 mark each)
• 20 Short Answer Questions (2-3 marks each)
• 15 Multiple Choice Questions
• 10 Long Answer Questions (5 marks each)
• 10 Numerical Problems

All questions include detailed explanations for better understanding!

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• ✅ By Abhinav Sir
• ✅ © Abhinav Anand Maths