Science — CBSE Class 10 board question

(a) Identifying the reducing agent:

The reducing agent is the substance that loses oxygen (or gets oxidised).

(i) NH₃ — nitrogen in NH₃ gets oxidised to NO; NH₃ is the reducing agent.
(ii) H₂O — oxygen is removed from water; H₂O is the reducing agent.
(iii) CO — CO gains oxygen to form CO₂; CO is the reducing agent.
(iv) H₂ — hydrogen gets oxidised to form H₂O; H₂ is the reducing agent.

(b) Definition of a Redox Reaction:

A reaction in which one substance gets oxidised (gains oxygen / loses hydrogen) and another gets reduced (loses oxygen / gains hydrogen) simultaneously is called a redox reaction.

Example: In Fe₂O₃ + 3CO → 2Fe + 3CO₂, CO is oxidised and Fe₂O₃ is reduced.

Source: Chapter 1, Chemical Reactions and Equations

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• For (a), the reducing agent is always the one that loses electrons / gets oxidised — in oxygen-based terms, it is the substance that gains oxygen or helps reduce the other substance. Students often confuse reducing agent with the substance being reduced; remember: the reducing agent itself gets oxidised.
• For (b), the examiner expects both terms (oxidation and reduction) defined together in one sentence to establish the "redox" concept, with an example.
• Stick to oxygen gain/loss language as the question specifies it.