CDS (I) 2021 General Knowledge · Question 98 of 120

Correct — A, Sulphur. Silver left in the open slowly dulls and blackens, and the black coating is silver sulphide, Ag₂S. It forms because air carries traces of hydrogen sulphide, and silver reacts with that sulphur compound at room temperature. Silver is a noble metal and resists ordinary oxidation, so oxygen is not what blackens it — jewellers rely on that, which is why silver keeps its shine indoors in clean dry air and loses it fast near cooking gas, boiled eggs, rubber bands or industrial fumes. The layer is thin and self-limiting: once formed, it protects the metal beneath, which is why a tarnished spoon can be cleaned back to brightness without losing measurable silver. The everyday check on this is that the same air turns copper green and iron reddish-brown, so each metal is being attacked by something different.

• (b)Oxygen — The instinctive answer and the wrong one. Silver sits low in the reactivity series and does not form an oxide layer in air at room temperature; if oxygen were the culprit the coating would be silver oxide, not the sulphide that analysis finds.
• (c)Carbon dioxide — Carbon dioxide with moisture is what corrodes copper, giving the green basic carbonate seen on old copper roofs and statues. Silver does not form a carbonate coating in air.
• (d)Phosphorus — There is no free phosphorus in ordinary air to react with. Phosphorus is stored under water precisely because it is too reactive to be left exposed.

Corrosion is the slow chemical attack of a metal by the substances around it, and each metal has its own signature product. Iron with oxygen and water gives hydrated iron oxide, which is rust; copper with moist carbon dioxide gives a green basic carbonate; silver with traces of hydrogen sulphide gives a black sulphide. The dulling of silver has its own name, tarnishing, and it is a chemical change because a new substance is formed.

The option list is a small corrosion quiz — one gas for each of the common school examples plus one element that is not in air at all. Match each to the metal it actually attacks and the answer falls out by elimination. The pull towards oxygen is strong because 'reacts with air' and 'oxidises' are treated as synonyms in casual speech; the fact to hold on to is that silver is among the least reactive metals and that the black product is a sulphide. One thing to notice on the printed page is that the stem reads 'Which one of the following substances do silver articles react with', where 'do' should be 'does' after 'which one'. The slip is in the grammar, not in the chemistry.

• The black coating on tarnished silver is silver sulphide, Ag₂S.
• It forms by reaction with traces of hydrogen sulphide in the air, not with oxygen.
• Silver sits low in the reactivity series, which is why it does not rust or oxidise in ordinary air.
• The tarnish layer is self-limiting and shields the silver underneath from further attack.
• Copper exposed to moist air turns green, iron turns reddish-brown, aluminium forms a colourless protective oxide film.

• Answering oxygen because the article is 'exposed to air'.
• Transferring copper's green carbonate coating to silver.
• Treating tarnishing as a physical change; a new compound is formed, so it is chemical.

Either as this direct 'what does it react with' item, or as 'name the compound formed', where the answer is Ag₂S.

• practice — not a real PYQ

  The green coating that forms on copper articles left in moist air is chiefly

  • (a)copper sulphide
  • (b)basic copper carbonate
  • (c)copper nitrate
  • (d)copper chloride

  Answer(b) basic copper carbonate — moist air supplies carbon dioxide and water, and the green patina of old copper roofs and statues is the result.

• practice — not a real PYQ

  Tarnishing of silver is best described as

  • (a)a physical change
  • (b)a chemical change
  • (c)sublimation
  • (d)a change of state

  Answer(b) a chemical change — a new substance, silver sulphide, is formed, and the original silver cannot be recovered by any physical means.