🔥 Combustion and Flame – MCQs for Class 8 Science

Part 3: Q51 – Q75

Q51. Which of the following is NOT a fossil fuel?

A) Coal
B) CNG
C) Biogas
D) Petroleum

✅ Correct Answer: C) Biogas

Explanation:

• A) Coal – Fossil fuel.
• B) CNG – Fossil fuel (compressed natural gas).
• C) ✔ Biogas is renewable, made from biomass, not a fossil fuel.
• D) Petroleum – Fossil fuel.

Q52. Which gas is produced in maximum quantity when fuels burn completely?

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

✅ Correct Answer: B) Carbon dioxide

Explanation:

• A) Carbon monoxide – Produced in incomplete combustion.
• B) ✔ Complete combustion of carbon → CO₂.
• C) Oxygen – Consumed, not produced.
• D) SO₂ – Only from sulphur-containing fuels.

Q53. Which of these is the main cause of acid rain?

A) Carbon dioxide
B) Carbon monoxide
C) Sulphur dioxide and nitrogen oxides
D) Methane

✅ Correct Answer: C) Sulphur dioxide and nitrogen oxides

Explanation:

• A, B, D – Do not form strong acids.
• C) ✔ SO₂ and NOₓ dissolve in rainwater → sulphuric/nitric acid.

Q54. Which factor is NOT necessary for combustion?

A) Fuel
B) Heat (ignition temperature)
C) Oxygen
D) Carbon dioxide

✅ Correct Answer: D) Carbon dioxide

Explanation:

• A, B, C – Essential for fire.
• D) ✔ CO₂ extinguishes fire, not needed.

Q55. Which is the safest gas used as domestic cooking fuel?

A) CNG
B) LPG
C) Biogas
D) Kerosene vapour

✅ Correct Answer: B) LPG

Explanation:

• A) CNG – Vehicles.
• B) ✔ LPG is efficient, clean, and widely used at home.
• C) Biogas – Used in rural areas, not common city homes.
• D) Kerosene vapour – Polluting and risky.

Q56. Which fire cannot be extinguished by water?

A) Paper fire
B) Cloth fire
C) Electrical fire
D) Wood fire

✅ Correct Answer: C) Electrical fire

Explanation:

• A, B, D – Water can extinguish.
• C) ✔ Water conducts electricity, unsafe for electrical fires.

Q57. Which zone of a candle flame is faintly luminous and moderately hot?

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

✅ Correct Answer: B) Middle zone

Explanation:

• A) Outermost – Blue, hottest.
• B) ✔ Middle yellow zone → partial combustion, luminous.
• C) Innermost – Least hot, unburnt vapours.
• D) Wick – Not a flame zone.

Q58. Why is CNG called an eco-friendly fuel?

A) It is cheap
B) Produces less smoke and pollutants
C) Found in large reserves
D) Odourless

✅ Correct Answer: B) Produces less smoke and pollutants

Explanation:

• A, C, D – Not the main reason.
• B) ✔ CNG burns cleanly with less harmful gases.

Q59. Which is a characteristic of incomplete combustion?

A) Blue flame, high heat
B) Yellow flame, soot formation
C) Smokeless burning
D) Full oxidation of fuel

✅ Correct Answer: B) Yellow flame, soot formation

Explanation:

• A, C, D – Indicate complete combustion.
• B) ✔ Incomplete combustion = yellow flame, carbon particles.

Q60. Which fuel among the following is renewable?

A) Biogas
B) Petrol
C) Coal
D) Diesel

✅ Correct Answer: A) Biogas

Explanation:

• A) ✔ Biogas is renewable, produced from cow dung/biomass.
• B, C, D – Fossil fuels, non-renewable.

Q61. Which type of flame is formed when oxygen supply is sufficient?

A) Yellow flame
B) Blue flame
C) Orange flame
D) Red flame

✅ Correct Answer: B) Blue flame

Explanation:

• A, C, D – Appear when oxygen is insufficient.
• B) ✔ Blue flame = complete combustion, hottest.

Q62. Which gas is released in dangerous amounts inside a closed room if coal is burnt?

A) CO₂
B) CO
C) CH₄
D) O₂

✅ Correct Answer: B) CO

Explanation:

• A) CO₂ – Not deadly in small amounts.
• B) ✔ Carbon monoxide is poisonous, causes suffocation.
• C) CH₄ – Not released during burning.
• D) O₂ – Consumed, not produced.

Q63. Which is NOT a type of combustion?

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

✅ Correct Answer: D) Evaporation

Explanation:

• A, B, C – Types of combustion.
• D) ✔ Evaporation is a physical change, not combustion.

Q64. Which of the following is an inflammable substance?

A) Glass
B) Iron
C) Petrol
D) Stone

✅ Correct Answer: C) Petrol

Explanation:

• A, B, D – Non-combustible.
• C) ✔ Petrol burns easily, highly inflammable.

Q65. Which gas is used in fire extinguishers for electrical fires?

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

✅ Correct Answer: B) Carbon dioxide

Explanation:

• A) Oxygen – Increases fire.
• B) ✔ CO₂ displaces oxygen and does not conduct electricity.
• C, D – Not used in extinguishers.

Q66. Which statement is true about calorific value?

A) Energy produced on burning 1 g fuel
B) Energy produced on burning 1 kg fuel
C) Energy produced on burning 1 L fuel
D) Energy produced on burning 1 mole fuel

✅ Correct Answer: B) Energy produced on burning 1 kg fuel

Explanation:

• A, C, D – Incorrect definitions.
• B) ✔ Correct definition: energy released per kilogram of fuel.

Q67. Which zone of the flame is blue and least luminous?

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

✅ Correct Answer: A) Outermost zone

Explanation:

• A) ✔ Outermost zone = complete combustion, blue, less luminous.
• B, C, D – Different zones.

Q68. Which fuel is used in thermal power stations mainly in India?

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

✅ Correct Answer: C) Coal

Explanation:

• A, B, D – Not used on large scale.
• C) ✔ Coal is the main fuel for power plants in India.

Q69. Which combustion is rusting of iron an example of?

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

✅ Correct Answer: A) Slow combustion

Explanation:

• A) ✔ Rusting is very slow oxidation → no flame.
• B, C, D – Not applicable.

Q70. Which gas is added to LPG for detecting leakage?

A) CO₂
B) SO₂
C) Ethyl mercaptan
D) Methane

✅ Correct Answer: C) Ethyl mercaptan

Explanation:

• A, B, D – Not added.
• C) ✔ Strong smell helps detect leakage.

Q71. Which flame is preferred for welding metals?

A) Yellow flame
B) Blue flame
C) Orange flame
D) Red flame

✅ Correct Answer: B) Blue flame

Explanation:

• A, C, D – Less hot flames.
• B) ✔ Blue flame is hottest, used for welding.

Q72. Which type of combustion is burning of camphor without a flame?

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

✅ Correct Answer: C) Flameless combustion

Explanation:

• A, B, D – Not suitable.
• C) ✔ Camphor burns with a weak flame, almost flameless.

Q73. Which of these fuels is used in vehicles to reduce air pollution?

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

✅ Correct Answer: C) CNG

Explanation:

• A, B, D – Produce more smoke.
• C) ✔ CNG is eco-friendly, used in vehicles.

Q74. Which gas is responsible for suffocation in mines due to incomplete combustion?

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

✅ Correct Answer: C) Carbon monoxide

Explanation:

• A, B, D – Not deadly in small amounts.
• C) ✔ CO is poisonous, binds haemoglobin.

Q75. Which part of the flame does goldsmiths use for melting gold and silver?

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

✅ Correct Answer: B) Outermost zone

Explanation:

• A, C, D – Less hot.
• B) ✔ Goldsmiths use blue outermost zone (hottest) for melting metals.

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