Silver articles become black after some time when exposed to air because
- A.silver gets oxidized to silver oxide
- B.silver reacts with moist carbon dioxide in the air to form silver carbonate
- C.silver reacts with sulphur in the air to form a coating of silver sulphide
- D.silver reacts with nitrogen oxides in the air to form silver nitrate
Correct answer
Explanation
The correct answer is C, silver reacts with sulphur in the air to form a coating of silver sulphide. The black layer on old silver is silver sulphide (Ag₂S).
Air contains traces of sulphur compounds, mainly hydrogen sulphide (H₂S) from burning fuels and decaying matter. Silver reacts slowly with them and forms a thin black film of silver sulphide; this is called tarnishing, a kind of corrosion. The film can be removed by polishing, or by placing the article in hot salt or baking soda solution with aluminium foil. Copper, by contrast, gains a green coat of basic copper carbonate, and iron forms reddish-brown rust.
A is wrong: silver is a noble metal and is not oxidised by the oxygen of air at room temperature.
B is wrong: the green carbonate coating forms on copper, not silver.
D is wrong: silver nitrate is made by dissolving silver in nitric acid; it does not form from air.
Exam tip: silver turns black (Ag₂S), copper turns green (basic copper carbonate), iron rusts reddish-brown (hydrated iron oxide).
Practice Questions
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