Oxidation cannot occur without simultaneous __________.

• A. Hydrolysis
• B. Combustion
• C. Reduction
• D. Neutralization

Answer: (C)

At the heart of oxidation-reduction reactions is electron transfer. Oxidation is the loss of electrons, but those electrons must be accepted by another species. So oxidation cannot occur without simultaneous reduction—the species that gains electrons becomes reduced. This pairing keeps electron accounting balanced and defines a redox couple: the donor is the reducing agent and is oxidized; the acceptor is the oxidizing agent and is reduced. For example, in rusting, iron loses electrons and is oxidized while oxygen gains those electrons and is reduced. The other processes listed—hydrolysis, neutralization, and combustion—are different types of reactions (water-driven, acid–base proton transfer, and rapid oxidation with heat, respectively) and do not capture the necessary electron-transfer pairing that defines redox.