Generated by claude-sonnet-4-6 · 2026-06-15 06:48 · grounding rag
Model Answer
(b)(i) When copper powder is heated in air, it reacts with oxygen to form copper oxide (CuO), which is black in colour. This is an oxidation reaction:
$$2Cu + O_2 \xrightarrow{\Delta} 2CuO$$
(b)(ii) When hydrogen gas is passed over the hot black copper oxide (CuO), it gets reduced back to copper, regaining its original shiny brown colour:
$$CuO + H_2 \xrightarrow{\Delta} Cu + H_2O$$
Source: Chapter 1, Chemical Reactions and Equations
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Explanation
- (b)(i): Examiners expect students to identify this as oxidation (gain of oxygen) and name the product as copper(II) oxide / CuO. The equation is a bonus but helps secure the mark.
- (b)(ii): The reverse process is reduction (removal of oxygen using hydrogen). Students must state that hydrogen is passed over CuO and that copper is restored. This concept directly links to Q17 of the exercises ("shiny brown coloured element X") and the oxidation-reduction topic in Chapter 1.