Which of the following substances is not a reducing agent?

Analyze the question: A reducing agent is a substance that loses (donates) electrons and is itself oxidised, so it reduces another substance. We need the option that does not do this.

Concept:

• Reducing agents are oxidised: their oxidation number increases, or they remove oxygen from or add hydrogen to another substance.
• Oxidising agents are reduced: they gain electrons, and oxygen is the classic example.

Solution, option by option:

• A. C (carbon): It is used to reduce metal oxides, e.g. in the blast furnace and when extracting zinc or lead. ZnO + C → Zn + CO Carbon goes from 0 to +2, so it is oxidised and acts as a reducing agent.

• B. CO (carbon monoxide): It reduces iron(III) oxide in the blast furnace. Fe₂O₃ + 3CO → 2Fe + 3CO₂ Carbon goes from +2 to +4, so it is oxidised and acts as a reducing agent.

• C. O₂ (oxygen): It typically gains electrons, going from 0 to −2, so it is reduced. This makes it an oxidising agent, not a reducing agent.

• D. H₂ (hydrogen): It reduces hot copper(II) oxide. CuO + H₂ → Cu + H₂O Hydrogen goes from 0 to +1, so it is oxidised and acts as a reducing agent.

Answer: The substance that is not a reducing agent is O₂, which is option C.

Tip: Remember "OIL RIG": Oxidation Is Loss, Reduction Is Gain (of electrons). Oxygen and the halogens are the typical oxidising agents. Carbon, carbon monoxide, hydrogen and reactive metals are the typical reducing agents.