Copper oxide is a black solid. When it is added to dilute hydrochloric acid with stirring, the black solid dissolves and the solution turns blue-green. Write the chemical equation for this reaction and use the nature of the products to explain how metallic oxides should be classified chemically.
Generated by claude-sonnet-4-6 · 2026-06-26 01:04 · grounding rag
Model Answer
Chemical equation:
$$\text{CuO (s) + 2HCl (aq)} \rightarrow \text{CuCl}_2\text{(aq) + H}_2\text{O (l)}$$
The black copper oxide dissolves to form copper chloride (which gives the blue-green colour) and water. Copper chloride is a salt, and water is formed — these are the products of a reaction between a base and an acid.
Since copper oxide reacts with hydrochloric acid to form salt and water, it behaves as a base. Therefore, metallic oxides should be classified as basic oxides (also called basic in nature). Some metallic oxides, such as aluminium oxide and zinc oxide, react with both acids and bases and are called amphoteric oxides.
Source: Chapter 3, Section 3.2.1
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Explanation
- Examiners award 1 mark for the correct balanced equation, 1 mark for identifying the products as salt + water (indicating a base–acid reaction), and 1 mark for the correct classification: metallic oxides are basic oxides (with the amphoteric example as a bonus/completing point).
- Write the equation with state symbols for full credit.
- The key phrase the examiner looks for is "metal oxides are basic in nature" — this must appear explicitly.
- Mentioning amphoteric oxides (Al₂O₃, ZnO) shows a more complete answer and is mentioned directly in the textbook in this context.