Seema took a small amount of copper powder in a china dish and heated it. What changes will she observe on heating ? When H₂ gas is passed over this heated substance, what visible changes will be seen in it ? Give the chemical equations of the reactions involved, along with the names and colours of the products in each case.
Generated by claude-sonnet-4-6 · 2026-06-14 10:31 · grounding rag
Model Answer
Observation on heating copper powder:
The surface of copper powder becomes coated with a black substance. This happens because copper reacts with oxygen in air to form copper(II) oxide (CuO), which is black.
$$2\text{Cu} + \text{O}_2 \xrightarrow{\Delta} 2\text{CuO}$$
(Copper — reddish-brown; Copper(II) oxide — black)
On passing H₂ gas over heated CuO:
The black coating turns brown as copper is reformed. CuO loses oxygen (gets reduced) and H₂ gains oxygen (gets oxidised). This is a redox reaction.
$$\text{CuO} + \text{H}_2 \xrightarrow{\Delta} \text{Cu} + \text{H}_2\text{O}$$
(Copper — brown/reddish; Water — colourless)
Source: Chapter 1, Section 1.2.5 (Oxidation and Reduction)
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Explanation
- Examiners expect both observations (black coating, then brown colour) clearly stated.
- Both balanced equations with heat arrow are mandatory for full marks.
- Name and colour of each product must be mentioned — this is a direct instruction in the question.
- Mentioning oxidation/reduction briefly adds value but is not compulsory for 3 marks.