Which of the following reactants is the best indicator that a chemical reaction is an oxidation-reduction (redox) reaction?

• A. Water (H_2O)
• B. Sodium chloride (NaCl)
• C. Elemental oxygen (O_2)
• D. Carbon dioxide (CO_2)

Answer: (C)

Verified step by step guidance

1. Understand that an oxidation-reduction (redox) reaction involves the transfer of electrons between species, resulting in changes in oxidation states.
2. Identify that a good indicator of a redox reaction is the presence of a reactant that can either gain or lose electrons, typically an element in its elemental form or a species known for changing oxidation states.
3. Analyze each reactant: Water (H_2O) and carbon dioxide (CO_2) are stable compounds with fixed oxidation states; sodium chloride (NaCl) is an ionic compound with ions already in their common oxidation states.
4. Recognize that elemental oxygen (O_2) is a diatomic molecule in its elemental form, which commonly acts as an oxidizing agent by accepting electrons and undergoing reduction.
5. Conclude that the presence of elemental oxygen (O_2) as a reactant strongly suggests a redox reaction because it participates in electron transfer, changing oxidation states during the reaction.