State giving reason for the change in appearance observed when each of the following metal is exposed to atmospheric air for some time : (i) Silver, (ii) Copper and (iii) Iron
Generated by claude-sonnet-4-6 · 2026-06-15 06:50 · grounding rag
Model Answer
(i) Silver turns black when exposed to air. This is because silver reacts with sulphur present in the air to form a black coating of silver sulphide (Ag₂S).
(ii) Copper loses its shiny brown surface and gains a green coating. This is because copper reacts with moist carbon dioxide in the air to form basic copper carbonate (green).
(iii) Iron develops a brown flaky coating called rust when exposed to moist air. Iron reacts with oxygen and moisture to form hydrated iron oxide (rust, Fe₂O₃·xH₂O).
Source: Chapter 3, Section 3.5 Corrosion; Chapter 1, Section 1.3.1
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Explanation
- Each metal = 1 mark: change in appearance + reason (chemical cause).
- Key terms examiners expect: silver sulphide (black), basic copper carbonate (green), rust/hydrated iron oxide (brown flaky).
- Don't just say "reacts with air" — name the specific substance (sulphur, CO₂ + moisture, O₂ + moisture) and the product formed. That's where the mark lies.