Which of the following situations most clearly demonstrates a key characteristic of a redox reaction?
Correct answer: B. Hydrogen gas reacting with chlorine to form hydrogen chloride gas
- A. Water boiling to steam
- B. Hydrogen gas reacting with chlorine to form hydrogen chloride gas
- C. Sodium chloride dissolving in water
- D. Ethanol evaporating at room temperature
Explanation
Hydrogen goes from zero to plus 1 and chlorine from zero to minus 1, so electrons are transferred and oxidation numbers change, which is the defining feature of a redox reaction. Boiling, dissolving and evaporating are physical changes in which no oxidation number moves at all. Checking oxidation numbers on both sides is the reliable test.
This question appeared on the UHS MDCAT 2025 paper, which you can sit online with every answer explained.
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About Redox Reactions
Redox reactions involve oxidation and reduction occurring together, with electrons transferred between species or oxidation numbers changing. The topic covers assigning oxidation states, identifying oxidizing and reducing agents, balancing redox equations by oxidation number or half-reaction methods, and connecting chemical changes with galvanic and electrolytic cells.
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