How is a redox reaction defined?

• A. A reaction where one element replaces another
• B. A reaction involving electron transfer between reactants
• C. A reaction forming water and salt
• D. A reaction that breaks down a compound

Answer: (B)

Electron transfer between reactants is the defining feature of a redox reaction. In redox processes, oxidation states change because electrons are moved from one species to another: one part of the system loses electrons (oxidation) and another part gains those electrons (reduction). Since electrons are conserved, both processes occur together in the same reaction.

A classic example is zinc metal reacting with copper(II) ions. Zinc loses electrons to become Zn2+, while Cu2+ gains those electrons to become copper metal. The movement of electrons from zinc to copper shows exactly what makes a reaction redox.

To recognize redox, look for changes in oxidation states of the elements involved. If you can track electrons being transferred, you’re observing a redox process. Some reactions involve bond breaking or formation, or changes in substance composition without an overall electron transfer, and those are not redox reactions.