🔥 Combustion and Flame – MCQs for Class 8 Science

Part 2: Q26 – Q50

Q26. Which of the following is an example of slow combustion?

A) Burning of wood
B) Respiration in living beings
C) Burning of LPG in stove
D) Explosion of crackers

✅ Correct Answer: B) Respiration in living beings

Explanation:

• A) Burning of wood – Rapid combustion.
• B) ✔ Respiration is slow oxidation releasing energy.
• C) LPG in stove – Rapid combustion.
• D) Crackers – Explosion.

Q27. Which of these can ignite spontaneously at room temperature?

A) Paper
B) Phosphorus
C) Wood
D) Charcoal

✅ Correct Answer: B) Phosphorus

Explanation:

• A, C, D – Require ignition source.
• B) ✔ White phosphorus has very low ignition temperature.

Q28. What is the unit of calorific value of a fuel?

A) J/kg
B) kJ/kg
C) J/g
D) kJ/L

✅ Correct Answer: B) kJ/kg

Explanation:

• A) J/kg – Too small a unit.
• B) ✔ Standard unit = kilojoules per kilogram.
• C, D – Incorrect.

Q29. Which part of the candle flame is least hot?

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

✅ Correct Answer: C) Innermost zone

Explanation:

• A) Outermost – Hottest.
• B) Middle – Moderately hot.
• C) ✔ Innermost zone contains unburnt vapours → least hot.
• D) Wick – Not part of flame zones.

Q30. Which type of flame is best for laboratory heating?

A) Yellow luminous flame
B) Blue non-luminous flame
C) Orange flame
D) Red flame

✅ Correct Answer: B) Blue non-luminous flame

Explanation:

• A, C, D – Less hot and smoky.
• B) ✔ Blue flame is hottest, smokeless, ideal for heating.

Q31. Which fuel is called a fossil fuel?

A) Wood
B) CNG
C) Biogas
D) Ethanol

✅ Correct Answer: B) CNG

Explanation:

• A) Wood – Biomass.
• B) ✔ CNG (Compressed Natural Gas) is derived from fossil deposits.
• C) Biogas – Renewable.
• D) Ethanol – Biofuel.

Q32. Which is NOT a property of an ideal fuel?

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

✅ Correct Answer: C) Produces more smoke

Explanation:

• A, B, D – Desirable qualities.
• C) ✔ Good fuel must not produce much smoke.

Q33. What does CNG stand for?

A) Common Natural Gas
B) Converted Natural Gas
C) Compressed Natural Gas
D) Controlled Natural Gas

✅ Correct Answer: C) Compressed Natural Gas

Explanation:

• A, B, D – Incorrect expansions.
• C) ✔ Correct full form of CNG.

Q34. Which of these fuels has the lowest ignition temperature?

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

✅ Correct Answer: D) Petrol

Explanation:

• A, B, C – Higher ignition temperatures.
• D) ✔ Petrol catches fire easily, very low ignition temperature.

Q35. Which factor does NOT affect combustion?

A) Presence of oxygen
B) Ignition temperature
C) Presence of fuel
D) Colour of the flame

✅ Correct Answer: D) Colour of the flame

Explanation:

• A, B, C – Essential factors.
• D) ✔ Colour has no effect on combustion.

Q36. Which type of combustion is burning of matchstick?

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

✅ Correct Answer: A) Rapid combustion

Explanation:

• A) ✔ Matchstick burns quickly after striking.
• B) Spontaneous – No external flame needed.
• C) Explosion – Sudden combustion.
• D) Slow – Occurs without flame.

Q37. Why is water not used to control petrol fires?

A) Petrol is heavier than water
B) Petrol floats on water and spreads fire
C) Water evaporates quickly
D) Water reacts with petrol

✅ Correct Answer: B) Petrol floats on water and spreads fire

Explanation:

• A) False – petrol is lighter.
• B) ✔ Petrol floats, spreads fire.
• C, D – Incorrect reasons.

Q38. Which gas is present in the innermost zone of a candle flame?

A) Oxygen
B) Carbon dioxide
C) Unburnt vapours of wax
D) Nitrogen

✅ Correct Answer: C) Unburnt vapours of wax

Explanation:

• A, B, D – Not correct.
• C) ✔ Innermost zone has unburnt wax vapours.

Q39. Which type of combustion occurs in burning of charcoal in air?

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

✅ Correct Answer: B) Rapid combustion

Explanation:

• A) Slow – Respiration, rusting.
• B) ✔ Charcoal burns rapidly with heat and light.
• C, D – Not suitable.

Q40. What is the calorific value of LPG approximately?

A) 25 kJ/kg
B) 55 kJ/g
C) 55 kJ/kg
D) 25 kJ/g

✅ Correct Answer: B) 55 kJ/g

Explanation:

• A, C, D – Wrong values/units.
• B) ✔ LPG ~55 kJ per gram.

Q41. Which fire extinguisher is safest for electrical fires?

A) Water extinguisher
B) Foam extinguisher
C) CO₂ extinguisher
D) Kerosene extinguisher

✅ Correct Answer: C) CO₂ extinguisher

Explanation:

• A) Water – Dangerous, conducts electricity.
• B) Foam – Suitable for oil, not electrical fires.
• C) ✔ CO₂ does not conduct, safest.
• D) Kerosene – Would worsen fire.

Q42. Which zone of flame produces light but less heat?

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

✅ Correct Answer: B) Middle luminous zone

Explanation:

• A) Innermost – Least hot, no light.
• B) ✔ Bright yellow luminous zone.
• C) Outermost – Hottest zone.
• D) Wick – Not a flame zone.

Q43. Which is a characteristic of complete combustion?

A) Yellow flame, soot formed
B) Blue flame, more heat produced
C) Smoke formation
D) Production of carbon monoxide

✅ Correct Answer: B) Blue flame, more heat produced

Explanation:

• A, C, D – Indicate incomplete combustion.
• B) ✔ Blue flame, clean burning = complete combustion.

Q44. Which fuel is used in domestic gas stoves in India?

A) CNG
B) LPG
C) Coal gas
D) Biogas

✅ Correct Answer: B) LPG

Explanation:

• A) CNG – Vehicles.
• B) ✔ Liquefied Petroleum Gas = common household fuel.
• C) Coal gas – Old use.
• D) Biogas – Rural areas.

Q45. Which type of combustion is burning of crackers?

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

✅ Correct Answer: C) Explosion

Explanation:

• A, B, D – Not suitable.
• C) ✔ Crackers burn suddenly with light, heat, sound = explosion.

Q46. Which fuel has the highest calorific value among these?

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

✅ Correct Answer: C) LPG

Explanation:

• A, B, D – Lower calorific value.
• C) ✔ LPG ~55 kJ/g, highest here.

Q47. Which is NOT an example of rapid combustion?

A) Burning of paper
B) Burning of kerosene
C) Burning of biogas
D) Rusting of iron

✅ Correct Answer: D) Rusting of iron

Explanation:

• A, B, C – Examples of rapid combustion.
• D) ✔ Rusting = slow oxidation, no flame.

Q48. Which of these produces maximum smoke while burning?

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

✅ Correct Answer: B) Diesel

Explanation:

• A, C, D – Cleaner fuels.
• B) ✔ Diesel produces black smoke.

Q49. Which is the hottest zone of flame?

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

✅ Correct Answer: C) Outermost

Explanation:

• A, B – Cooler.
• C) ✔ Outermost zone = complete combustion, hottest.
• D) Wick – Not a zone.

Q50. Which gas is dangerous when inhaled due to incomplete combustion of fuels?

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

✅ Correct Answer: C) Carbon monoxide

Explanation:

• A) CO₂ – Not immediately fatal.
• B) Oxygen – Life-supporting.
• C) ✔ CO is poisonous, binds haemoglobin.
• D) Methane – Explosive but not from incomplete combustion.

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