🔥 Combustion and Flame – MCQs for Class 8 Science

Part 4: Q76 – Q100

Q76. Which of the following is NOT an example of spontaneous combustion?

A) White phosphorus catching fire in air
B) Burning of coal in air
C) Explosion of cracker
D) Burning of matchstick

✅ Correct Answer: B) Burning of coal in air

Explanation:

• A) White phosphorus – ✔ Burns on its own at room temperature.
• B) Coal – Needs ignition → not spontaneous.
• C) Crackers – Explosion, not spontaneous.
• D) Matchstick – Needs striking → not spontaneous.

Q77. Which type of fire extinguisher is used for oil fires?

A) Water
B) Foam
C) Sand
D) CO₂

✅ Correct Answer: B) Foam

Explanation:

• A) Water – Spreads oil fire.
• B) ✔ Foam covers surface of oil, cuts oxygen.
• C) Sand – Works for small oil spills, not large.
• D) CO₂ – Works but foam is better for oils.

Q78. Which part of the candle flame is least luminous?

A) Outermost zone
B) Middle zone
C) Innermost zone
D) Wick

✅ Correct Answer: A) Outermost zone

Explanation:

• A) ✔ Blue outermost zone, least luminous but hottest.
• B) Middle – Yellow, luminous.
• C) Innermost – Dark, less bright.
• D) Wick – Not a zone of flame.

Q79. Which fuel is most suitable for rural biogas plants?

A) LPG
B) Cow dung
C) Petrol
D) Kerosene

✅ Correct Answer: B) Cow dung

Explanation:

• A, C, D – Fossil fuels, not renewable.
• B) ✔ Cow dung → anaerobic digestion → biogas.

Q80. What is the main gas present in biogas?

A) Carbon dioxide
B) Methane
C) Nitrogen
D) Oxygen

✅ Correct Answer: B) Methane

Explanation:

• A) CO₂ – Present but lesser.
• B) ✔ Methane (50–70%) is main gas.
• C, D – Not major components.

Q81. Which is NOT a property of CNG?

A) Clean fuel
B) Produces less smoke
C) Easily liquefied at room temperature
D) Used in vehicles

✅ Correct Answer: C) Easily liquefied at room temperature

Explanation:

• A, B, D – True.
• C) ✔ CNG requires very high pressure to compress, not liquefied easily.

Q82. Which type of combustion takes place in burning of a magnesium ribbon?

A) Slow combustion
B) Rapid combustion
C) Explosion
D) Spontaneous combustion

✅ Correct Answer: B) Rapid combustion

Explanation:

• A, C, D – Not correct.
• B) ✔ Magnesium ribbon burns quickly with bright light.

Q83. Which device is used to measure the calorific value of fuels?

A) Thermometer
B) Barometer
C) Bomb calorimeter
D) Hygrometer

✅ Correct Answer: C) Bomb calorimeter

Explanation:

• A, B, D – Measure unrelated properties.
• C) ✔ Standard instrument to calculate calorific value.

Q84. Which is the calorific value of wood approximately?

A) 17–20 kJ/g
B) 55 kJ/g
C) 45 kJ/g
D) 50 kJ/g

✅ Correct Answer: A) 17–20 kJ/g

Explanation:

• A) ✔ Wood has lowest calorific value compared to other fuels.
• B, C, D – Values of LPG, petrol, etc.

Q85. Which zone of the flame does not produce light?

A) Outermost zone
B) Middle luminous zone
C) Innermost dark zone
D) Wick

✅ Correct Answer: C) Innermost dark zone

Explanation:

• A, B – Produce light.
• C) ✔ Innermost zone has unburnt vapours, no light.
• D) Wick – Source of vapours.

Q86. Which gas is released in large amounts when forests catch fire?

A) Oxygen
B) Carbon dioxide
C) Carbon monoxide
D) Methane

✅ Correct Answer: B) Carbon dioxide

Explanation:

• A) Oxygen – Consumed, not produced.
• B) ✔ CO₂ released from burning trees.
• C) CO – Produced but lesser.
• D) Methane – Not main gas.

Q87. Which type of flame is luminous and yellow in colour?

A) Complete combustion flame
B) Incomplete combustion flame
C) Blue flame
D) Outer flame

✅ Correct Answer: B) Incomplete combustion flame

Explanation:

• A) Complete combustion – Blue flame.
• B) ✔ Incomplete combustion produces yellow, luminous flame.
• C, D – Not correct.

Q88. Which fuel produces maximum energy per gram?

A) LPG
B) Coal
C) Wood
D) Petrol

✅ Correct Answer: A) LPG

Explanation:

• A) ✔ LPG ~55 kJ/g, highest among common fuels.
• B, C, D – Lower calorific values.

Q89. Which zone of a candle flame is used by goldsmiths for melting metals?

A) Innermost zone
B) Middle luminous zone
C) Outermost non-luminous zone
D) Wick

✅ Correct Answer: C) Outermost non-luminous zone

Explanation:

• A, B, D – Not hot enough.
• C) ✔ Outermost zone is hottest, used in metalwork.

Q90. Which of these fuels is considered the cleanest?

A) LPG
B) Petrol
C) CNG
D) Diesel

✅ Correct Answer: C) CNG

Explanation:

• A, B, D – Cleaner than coal, but still produce smoke.
• C) ✔ CNG produces almost no smoke, very eco-friendly.

Q91. Which type of fire can be extinguished using water?

A) Electrical fire
B) Petrol fire
C) Coal/wood fire
D) Oil fire

✅ Correct Answer: C) Coal/wood fire

Explanation:

• A, B, D – Dangerous with water.
• C) ✔ Water cools and extinguishes solid fuel fires.

Q92. Which zone of a candle flame has partial combustion?

A) Outermost
B) Middle luminous
C) Innermost dark zone
D) Wick

✅ Correct Answer: B) Middle luminous

Explanation:

• A) Outermost – Complete combustion.
• B) ✔ Middle yellow zone → partial combustion.
• C) Innermost – Unburnt vapours.
• D) Wick – Fuel source.

Q93. Which factor is NOT required to maintain fire?

A) Fuel
B) Heat
C) Oxygen
D) Carbon dioxide

✅ Correct Answer: D) Carbon dioxide

Explanation:

• A, B, C – Essential factors.
• D) ✔ CO₂ extinguishes fire.

Q94. Which type of combustion occurs in burning coal in a chulha (slowly with smoke)?

A) Slow combustion
B) Rapid combustion
C) Incomplete combustion
D) Explosion

✅ Correct Answer: C) Incomplete combustion

Explanation:

• A) Slow – Rusting, respiration.
• B) Rapid – Quick burning.
• C) ✔ Coal in chulha burns incompletely → smoke.
• D) Explosion – Not correct.

Q95. Which is the approximate calorific value of coal?

A) 17–20 kJ/g
B) 25–30 kJ/g
C) 55 kJ/g
D) 45 kJ/g

✅ Correct Answer: B) 25–30 kJ/g

Explanation:

• A) Wood.
• B) ✔ Coal = 25–30 kJ/g.
• C, D – Other fuels.

Q96. Which gas is the main cause of deaths in closed rooms with burning charcoal?

A) CO₂
B) CO
C) SO₂
D) CH₄

✅ Correct Answer: B) CO

Explanation:

• A) CO₂ – Not immediately fatal.
• B) ✔ Carbon monoxide is poisonous.
• C, D – Not main culprits.

Q97. Which type of combustion produces sound?

A) Slow combustion
B) Rapid combustion
C) Spontaneous combustion
D) Explosion

✅ Correct Answer: D) Explosion

Explanation:

• A, B, C – Do not produce sound.
• D) ✔ Explosions release heat, light, sound suddenly.

Q98. Which gas is produced when sulphur burns in air?

A) CO₂
B) SO₂
C) H₂O vapour
D) NO₂

✅ Correct Answer: B) SO₂

Explanation:

• A, C, D – Not correct.
• B) ✔ Sulphur + oxygen → sulphur dioxide (SO₂).

Q99. Which is the most important characteristic of a good fuel?

A) High ignition temperature
B) Low calorific value
C) High calorific value
D) Produces more smoke

✅ Correct Answer: C) High calorific value

Explanation:

• A, B, D – Undesirable properties.
• C) ✔ High calorific value = more energy.

Q100. Which type of combustion is respiration in humans?

A) Explosion
B) Spontaneous combustion
C) Slow combustion
D) Rapid combustion

✅ Correct Answer: C) Slow combustion

Explanation:

• A, B, D – Not correct.
• C) ✔ Respiration = slow oxidation of glucose, energy released gradually.

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