Which change is exothermic? MCQ with Answer and Explanation

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Which change is exothermic?

A. Evaporation of water

B. Combustion of methane

C. Sublimation of camphor

D. Melting of ice

Answer: Option B

Solution (By JKExamLibrary)

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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Which of the following reactions is a neutralization reaction?

A. CaCO₃ → CaO + CO₂

B. 2Na + Cl₂ → 2NaCl

C. HCl + NaOH → NaCl + H₂O

D. Fe + CuSO₄ → FeSO₄ + Cu

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Correct Answer: Option C

Explanation:
Neutralization is the reaction of an acid and a base to form salt and water. HCl (acid) + NaOH (base) → NaCl (salt) + H₂O. Option A is combination; C is decomposition; D is displacement.

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Question #2

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The reaction CaO + H₂O → Ca(OH)₂ is a:

A. Redox

B. Decomposition

C. Displacement

D. Combination

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Correct Answer: Option D

Explanation:
Two reactants combine to form a single product, so combination (also called synthesis). No oxidation state change, so not redox. Decomposition is opposite.

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Question #3

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Teflon is a polymer of:

A. Tetrafluoroethene

B. Vinyl chloride

C. Styrene

D. Ethylene

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Correct Answer: Option A

Explanation:
Teflon (PTFE) is an addition polymer of tetrafluoroethene (CF₂=CF₂). The polymer has a linear chain of –CF₂–CF₂– units with high C–F bond strength, making it chemically inert and heat resistant. PVC is polymer of vinyl chloride (CH₂=CHCl). Polyethylene comes from ethylene. Polystyrene from styrene. Teflon is used for non-stick coatings, electrical insulation, and chemical containers.

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