Combustion and Flame — Class 8 Science NCERT Solutions (Free)
Free step-by-step NCERT solutions for Class 8 Science chapter "Combustion and Flame" — 5 important questions with detailed answers for CBSE board exam preparation.
TL;DR: Free step-by-step NCERT solutions for Class 8 Science chapter "Combustion and Flame" — 5 important questions with detailed answers for CBSE board exam…
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NCERT Solutions for Class 8 Science — Combustion and Flame. Step-by-step answers to all 5 textbook questions from this chapter, written for CBSE board preparation and free to use.
Combustion and Flame — All 5 Questions Solved
Q1. दहन क्या है और इसके लिए क्या आवश्यक है?
दहन वह प्रक्रिया है जिसमें कोई पदार्थ ऑक्सीजन के साथ अभिक्रिया करके ऊष्मा और प्रकाश उत्पन्न करता है। दहन के लिए तीन चीजें आवश्यक हैं: ईंधन, ऑक्सीजन और ताप। इन तीनों को दहन त्रिभुज कहा जाता है। इनमें से किसी एक के अभाव में दहन नहीं हो सकता।
Q2. ईंधन क्या है और इसके उदाहरण क्या हैं?
ईंधन वह पदार्थ है जो जलता है और ऊष्मा उत्पन्न करता है। कोयला, लकड़ी, पेट्रोल, डीजल, गैस आदि ईंधन हैं। ईंधन दो प्रकार के होते हैं: जीवाश्म ईंधन और गैर-जीवाश्म ईंधन। कोयला और पेट्रोल जीवाश्म ईंधन हैं। लकड़ी गैर-जीवाश्म ईंधन है।
Q3. दहन क्या प्रकार के होते हैं?
दहन के मुख्य प्रकार हैं: तीव्र दहन, विस्फोटक दहन और धीमा दहन। तीव्र दहन में तेजी से ऊष्मा और प्रकाश उत्पन्न होता है। विस्फोटक दहन अचानक होता है। धीमा दहन में ऊष्मा धीरे-धीरे उत्पन्न होती है जैसे ईंट की भट्टी में।
Q4. ज्वाला क्या है और इसके रंग को क्या प्रभावित करता है?
ज्वाला वह गैस है जो जलती है और प्रकाश उत्पन्न करती है। ज्वाला के रंग तापमान पर निर्भर करते हैं। पीली ज्वाला का तापमान कम होता है। नीली ज्वाला का तापमान अधिक होता है। ईंधन की गुणवत्ता भी ज्वाला के रंग को प्रभावित करती है।
Q5. अग्नि को नियंत्रित कैसे किया जाता है?
अग्नि को नियंत्रित करने के लिए दहन त्रिभुज में से किसी एक को हटाया जाता है। पानी डालने से ताप कम होता है। बालू या कंबल से ऑक्सीजन की आपूर्ति रोकी जाती है। ईंधन को हटा देने से भी आग बुझती है। अग्नि को नियंत्रित करने के लिए सही तरीका जानना जरूरी है।
Combustion and Flame — Practice MCQs with Answers
Attempt these 20 multiple-choice questions after working through the solutions above. Each carries the correct option and the reasoning behind it, so a wrong answer tells you which idea to revisit.
MCQ 1. What is the process of a substance reacting with oxygen to produce heat and light called?
- A. Respiration
- B. Digestion
- C. Combustion
- D. Evaporation
Answer: C. Combustion — Combustion is a chemical process in which a substance reacts with oxygen to give off heat and often light.
MCQ 2. Which of the following gases is essential for combustion to take place?
- A. Nitrogen
- B. Carbon dioxide
- C. Oxygen
- D. Hydrogen
Answer: C. Oxygen — Oxygen acts as a supporter of combustion; without it, most substances cannot burn.
MCQ 3. The lowest temperature at which a substance catches fire is called its:
- A. Melting point
- B. Boiling point
- C. Ignition temperature
- D. Critical temperature
Answer: C. Ignition temperature — Ignition temperature is the minimum temperature a combustible substance must reach to start burning.
MCQ 4. Which of the following is an inflammable substance?
- A. Stone
- B. Water
- C. Petrol
- D. Sand
Answer: C. Petrol — Inflammable substances have a very low ignition temperature and can easily catch fire with a flame.
MCQ 5. Why is water generally not used to extinguish fires involving electrical equipment?
- A. Water is a good conductor of electricity and can cause electrocution.
- B. Water increases the fire.
- C. Water reacts with electricity.
- D. Water is heavy.
Answer: A. Water is a good conductor of electricity and can cause electrocution. — Using water on electrical fires is dangerous because water conducts electricity, posing a risk of electric shock.
MCQ 6. Which zone of a candle flame is the hottest?
- A. Dark innermost zone
- B. Luminous middle zone
- C. Non-luminous outermost zone
- D. Blue zone near the wick
Answer: C. Non-luminous outermost zone — The non-luminous outermost zone of the flame has complete combustion due to ample oxygen supply and is thus the hottest part.
MCQ 7. What is the unit used to express the calorific value of a fuel?
- A. Joule (J)
- B. Kilojoule per gram (kJ/g)
- C. Kilojoule per kilogram (kJ/kg)
- D. Calorie (cal)
Answer: C. Kilojoule per kilogram (kJ/kg) — The calorific value of a fuel is the amount of heat energy produced on complete combustion of 1 kg of fuel, expressed in kJ/kg.
MCQ 8. What kind of combustion is observed when phosphorus burns in air at room temperature?
- A. Rapid combustion
- B. Spontaneous combustion
- C. Explosion
- D. Slow combustion
Answer: B. Spontaneous combustion — Spontaneous combustion occurs without the application of external heat, as the substance reaches its ignition temperature on its own.
MCQ 9. Which gas is released during incomplete combustion of fuels like coal and wood, and is very poisonous?
- A. Carbon dioxide
- B. Sulphur dioxide
- C. Carbon monoxide
- D. Nitrogen dioxide
Answer: C. Carbon monoxide — Incomplete combustion of carbon-based fuels produces carbon monoxide, which is a highly poisonous gas.
MCQ 10. A good fuel is one that:
- A. Has low calorific value
- B. Produces a lot of ash
- C. Is readily available and cheap
- D. Burns with a lot of smoke
Answer: C. Is readily available and cheap — A good fuel should be readily available, cheap, burn easily, and produce a large amount of heat without leaving undesirable substances.
MCQ 11. Why does a paper cup containing water not burn easily when a flame is applied to it?
- A. Paper is naturally fire-resistant.
- B. The water absorbs the heat, keeping the paper's temperature below its ignition temperature.
- C. Water forms a protective layer.
- D. Paper needs more oxygen to burn.
Answer: B. The water absorbs the heat, keeping the paper's temperature below its ignition temperature. — The heat supplied to the paper cup is transferred to the water, preventing the paper from reaching its ignition temperature and thus from burning.
MCQ 12. Which of the following is NOT a condition necessary for combustion?
- A. Presence of a combustible substance
- B. Presence of oxygen
- C. Reaching ignition temperature
- D. Presence of nitrogen
Answer: D. Presence of nitrogen — The three essential conditions for combustion are the presence of a combustible substance (fuel), oxygen, and reaching the ignition temperature. Nitrogen is not directly involved.
MCQ 13. How does a carbon dioxide (CO2) fire extinguisher work?
- A. It lowers the ignition temperature.
- B. It cools down the combustible substance.
- C. It cuts off the oxygen supply and cools the fuel.
- D. It adds water to the fire.
Answer: C. It cuts off the oxygen supply and cools the fuel. — CO2 is heavier than oxygen, so it forms a blanket around the fire, cutting off the oxygen supply and also cooling the fuel.
MCQ 14. What is the main environmental problem associated with the increased amount of carbon dioxide in the atmosphere due to burning of fuels?
- A. Acid rain
- B. Ozone layer depletion
- C. Global warming
- D. Water pollution
Answer: C. Global warming — Increased carbon dioxide traps heat radiation, leading to a rise in the Earth's average temperature, which is known as global warming.
MCQ 15. When a matchstick is rubbed against the rough surface of a matchbox, it catches fire. Which type of combustion is this an example of?
- A. Spontaneous combustion
- B. Explosion
- C. Rapid combustion
- D. Slow combustion
Answer: C. Rapid combustion — Rapid combustion is when a substance burns quickly and produces heat and light in a short period, often initiated by an external source like friction.
MCQ 16. Which of the following fuels has the highest calorific value?
- A. Wood
- B. Coal
- C. LPG
- D. Hydrogen
Answer: D. Hydrogen — Hydrogen gas has the highest calorific value among the given options, indicating it produces the most heat per unit mass.
MCQ 17. The dark innermost zone of a candle flame consists of:
- A. Completely burnt gases
- B. Unburnt carbon particles
- C. Unburnt wax vapours
- D. Hot air
Answer: C. Unburnt wax vapours — The innermost zone, also called the dark zone, is the least hot and consists of unburnt wax vapours that rise from the wick.
MCQ 18. Why is it dangerous to sleep in a room with burning coal in a closed fireplace?
- A. It produces too much heat.
- B. It consumes all the oxygen.
- C. It produces poisonous carbon monoxide gas.
- D. It causes an explosion.
Answer: C. It produces poisonous carbon monoxide gas. — Burning coal in a closed room results in incomplete combustion, producing highly poisonous carbon monoxide gas, which can be fatal.
MCQ 19. Acid rain is primarily caused by the release of which gases into the atmosphere when fuels are burnt?
- A. Carbon dioxide and Methane
- B. Sulphur dioxide and Nitrogen oxides
- C. Oxygen and Nitrogen
- D. Hydrogen and Helium
Answer: B. Sulphur dioxide and Nitrogen oxides — Burning of coal, petrol, and diesel releases sulphur dioxide and nitrogen oxides, which dissolve in rainwater to form acids, leading to acid rain.
MCQ 20. Which of the following is considered a fossil fuel?
- A. Wood
- B. LPG
- C. Solar energy
- D. Coal
Answer: D. Coal — Fossil fuels like coal, petroleum, and natural gas are formed from the decomposition of dead organisms over millions of years.
How to Use These Solutions
Attempt each question yourself first and only then compare with the worked answer. Marks in the CBSE board exam are awarded for the METHOD as much as the final result, so reproduce the steps rather than memorising the last line. Where a solution states a law, a formula or a definition, learn that wording — examiners look for it.
Related practice for this chapter: chapter MCQs, previous-year questions, revision notes and sample papers.