CDS (I) 2020 General Knowledge · Question 23 of 120

Correct — C, silver reacts with sulphur in the air to form a coating of silver sulphide. The stem and this option are NCERT's own sentences on corrosion, almost word for word — silver articles become black after some time when exposed to air, because the metal reacts with sulphur in the air to form a coating of silver sulphide. The compound is Ag₂S, a dense black solid, and the sulphur reaching the metal is mainly traces of hydrogen sulphide in the atmosphere. The film is thin and self-limiting, which is why tarnish dulls silverware without eating it away.

• (a)silver gets oxidized to silver oxide — Silver does not react with air even at red heat. Silver oxide exists but has to be made indirectly, by adding alkali to a soluble silver salt, and it decomposes above about 160 degrees Celsius.
• (b)silver reacts with moist carbon dioxide in the air to form silver carbonate — This is NCERT's sentence about copper, transplanted onto silver. Copper reacting with moist carbon dioxide is what produces the green coat of basic copper carbonate on a copper roof or vessel.
• (d)silver reacts with nitrogen oxides in the air to form silver nitrate — Silver nitrate is made by dissolving silver in nitric acid, not by exposure to trace nitrogen oxides in air. It is also a colourless crystalline solid, so it could not account for a black film.

Corrosion is the slow attack of a metal by substances in its surroundings, and each metal has its own signature product. Iron in moist air gives a brown flaky rust of hydrated iron oxide. Copper in moist carbon dioxide gains a green coat of basic copper carbonate. Silver in air containing sulphur compounds gains a black film of silver sulphide. Aluminium forms a colourless oxide layer that seals the surface and protects the metal beneath.

All four options are grammatically identical — silver plus something in the air giving a named compound — so the discrimination has to come from knowing which reagent silver actually responds to. Two checks help. Colour is one: the observed film is black, and of the four named products only the sulphide is black. Reactivity is the other: silver sits near the bottom of the activity series, low enough that oxygen alone does not touch it, and sulphur compounds are the exception that does. The copper line in option (b) is worth learning as a pair with the silver one, because papers routinely swap the two metals between the two reactions.

• NCERT records that silver articles turn black because silver reacts with sulphur in the air to form a coating of silver sulphide.
• The tarnish is Ag₂S, a dense black solid; the sulphur usually arrives as traces of hydrogen sulphide.
• Silver does not react with air even at red heat, so silver oxide is not the tarnish product.
• Copper in moist carbon dioxide gains a green coat of basic copper carbonate; iron in moist air acquires brown rust.
• The sulphide layer is self-limiting, protecting the silver underneath from further conversion.

• Choosing oxidation because blackening feels like oxidation; silver is unreactive towards oxygen.
• Swapping the copper and silver reactions, which the paper explicitly offers as option (b).
• Assuming any dark film on a metal must be an oxide.

As why silver goes black, as which substance silver reacts with, or as a match of metal to corrosion product.

• practice — not a real PYQ

  The green coating that forms on copper articles exposed to moist air for a long time is

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

  Answer(c) basic copper carbonate — formed when copper reacts with moist carbon dioxide in the air.

• practice — not a real PYQ

  Aluminium articles resist corrosion because

  • (a)aluminium does not react with oxygen
  • (b)a thin protective oxide layer forms on the surface
  • (c)aluminium is below hydrogen in the activity series
  • (d)aluminium reacts with nitrogen instead of oxygen

  Answer(b) a thin protective oxide layer forms on the surface — the layer is adherent and seals the metal beneath, unlike flaky rust on iron.