Which change is exothermic? MCQ with Answer and Explanation

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Which change is exothermic?

A. Melting of ice

B. Sublimation of camphor

C. Evaporation of water

D. Combustion of methane

Answer: Option D

Solution (By JKSSB Mock Tests)

Combustion of methane (CH₄ + 2O₂ → CO₂ + 2H₂O + heat) is an exothermic reaction, releasing heat to surroundings. Melting, evaporation, and sublimation are endothermic phase changes requiring heat absorption to overcome intermolecular forces. Exothermic reactions: burning, respiration, neutralization (usually), condensation, freezing. Endothermic: photosynthesis, cooking, melting, boiling.

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Question #1

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The main constituent of CNG is:

A. Propane

B. Butane

C. Methane

D. Ethane

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Correct Answer: Option C

Explanation:
CNG (Compressed Natural Gas) is primarily methane (CH₄). LPG is propane and butane. Methane is the lightest hydrocarbon. CNG is a cleaner fuel for vehicles.

This question belongs to: Science chemistry

Question #2

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Radioactive isotope used in dating ancient objects is:

A. Carbon-14

B. Uranium-235

C. Iodine-131

D. Cobalt-60

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Correct Answer: Option A

Explanation:
Carbon-14 dating is used to determine age of organic materials up to ~50,000 years based on its half-life of 5730 years. Cobalt-60 for cancer therapy, iodine-131 for thyroid, uranium-235 for nuclear fuel.

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Question #3

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The horizontal rows in the modern periodic table are called:

A. Periods

B. Families

C. Blocks

D. Groups

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Correct Answer: Option A

Explanation:
The periodic table has 7 horizontal rows called periods. Elements in the same period have the same number of electron shells. Properties change gradually across a period. Groups are vertical columns (18), where elements have similar valence electron configurations and thus similar chemical properties. Families are informal group names (e.g., halogens). Blocks (s, p, d, f) are based on the orbital in which the last electron enters.

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