Read the following and answer the questions that follow:
Ramesh is a jeweller who works with different metals daily. He notices that iron tools rust quickly if left damp, while his gold and silver ornaments stay shiny for years. Old copper bangles develop a green coating, which disappears when dipped in dilute acid. To make jewellery more durable, pure gold is mixed with copper or silver before shaping.
(i) Gold and silver resist corrosion under normal conditions while iron rusts. What does this tell you about the relative positions of these metals in the reactivity series? (1 mark)
(ii) What is the green coating on copper bangles? Why does it dissolve in dilute acid? (1 mark)
(iii) What is the term for the mixture obtained by combining gold with copper or silver? State one advantage of making this mixture over using pure gold. (1 mark)
(iv) A student claims that iron will rust if kept in completely dry air. Is this claim correct? Justify your answer. (1 mark)
Generated by claude-sonnet-4-6 · 2026-06-26 01:06 · grounding rag
Model Answer
(i) Gold and silver are low in the reactivity series (least reactive), so they do not react with moisture or oxygen easily. Iron is higher in the reactivity series and thus more reactive, making it prone to rusting.
(ii) The green coating is basic copper carbonate, formed when copper reacts with moist CO₂ in air. It is a base, so it reacts with and dissolves in dilute acid (neutralisation reaction).
(iii) The mixture is called an alloy. Advantage: Pure gold (24 carat) is very soft; alloying with copper or silver makes it harder and more suitable for making jewellery.
(iv) The claim is incorrect. Iron requires both air (oxygen) and moisture to rust. In completely dry air, no moisture is present, so rusting will not occur (as shown by the iron nail in dry air remaining rust-free).
Source: Chapter 3, Section 3.5 Corrosion; Do You Know box (Alloys)
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Explanation
- (i) Examiners expect mention of reactivity series positions — gold/silver are least reactive (bottom), iron is above them.
- (ii) The exact name "basic copper carbonate" is required; saying "it dissolves because it is basic/alkaline" earns the mark.
- (iii) "Alloy" is the keyword. The advantage must relate to hardness/durability, not just appearance.
- (iv) This is a direct application of Activity 3.14 — both air AND water are needed. Stating only one condition is insufficient.