Q1. [3] medium exam-ready
Explain why copper articles develop a green coating on their surface over time. Also explain the chemical nature of the substance that tarnishes silver articles and that which tarnishes copper articles.
Generated by claude-sonnet-4-6 · 2026-06-26 01:05 · grounding rag
Model Answer
Copper reacts with moist carbon dioxide in the air, forming a green coating of basic copper carbonate on its surface. This is a corrosion reaction.
- The substance that tarnishes silver is silver sulphide — it is a sulphide (formed by reaction of silver with sulphur present in air). It is chemically a salt.
- The substance that tarnishes copper is basic copper carbonate — it is chemically a basic salt (or carbonate in nature).
Source: Chapter 3, Section 3.5 Corrosion
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Explanation
- The question has two parts: (i) why copper develops a green coat (the reason/reaction), and (ii) the chemical nature of the tarnishing substances for both silver and copper. Address both clearly.
- Key fact: silver → silver sulphide (sulphide salt); copper → basic copper carbonate (basic salt/carbonate).
- Don't confuse: silver turns black (silver sulphide), copper turns green (basic copper carbonate). Examiners check whether you correctly name both compounds and their chemical nature.
- The word "basic copper carbonate" is expected verbatim from the textbook.