Answer: Reducing agent.
- A Oxidising agent
- B Reducing agent
- C Both oxidising and reducing agent
- D Catalyst
Correct answer: B. Reducing agent
Explanation: S₂O₃²⁻ loses electrons (S goes from +2 to +2.5 on average in S₄O₆²⁻); I₂ gains electrons (reduced to I⁻). Therefore Na₂S₂O₃ is the reducing agent.
Electron transfer in a redox reaction: zinc loses electrons (oxidised, acts as reducing agent) and copper(II) ions gain those same electrons (reduced, acts as oxidising agent).
Concept context
Master electron transfer in chemistry: assign oxidation states, balance half-reactions, identify oxidising and reducing agents, and connect redox to everyday reactions.