Q1. [3] medium thorough-understanding
When copper oxide is heated with hydrogen gas, the black coating turns brown. In this reaction, identify which substance is oxidised and which is reduced, and explain why, using the definitions of oxidation and reduction in terms of oxygen.
Generated by claude-sonnet-4-6 · 2026-06-26 00:55 · grounding rag
Model Answer
The reaction is:
$$\text{CuO} + \text{H}_2 \xrightarrow{\text{Heat}} \text{Cu} + \text{H}_2\text{O}$$
Hydrogen is oxidised because it gains oxygen (from CuO) to form water. Oxidation is defined as the gain of oxygen by a substance.
Copper(II) oxide is reduced because it loses oxygen to form copper. Reduction is defined as the loss of oxygen by a substance.
Since one reactant is oxidised and the other is reduced simultaneously, this is called a redox reaction.
Source: Chapter 1, Section 1.2.5 — Oxidation and Reduction
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Explanation
- 3 marks are split as: 1 mark for identifying the oxidised substance with reason, 1 mark for identifying the reduced substance with reason, 1 mark for the definitions (or the term "redox reaction").
- Always state the definition of oxidation/reduction explicitly — examiners look for "gains oxygen" / "loses oxygen" in the answer.
- Writing the chemical equation at the start is good practice and shows clarity.
- Do not confuse: hydrogen gains oxygen → oxidised; CuO loses oxygen → reduced. A common mistake is flipping these.