NDA (II) 2022 General Ability Test · Question 74 of 150

Correct — C, silver sulphide. Silverware left in the open dulls to a grey-black because the metal reacts with traces of hydrogen sulphide in the air to form a thin film of silver sulphide, Ag₂S, described in reference sources as a dense black solid. Sulphur compounds, not oxygen, do the work — plain air attacks silver only very slowly. The film is self-limiting, which is why the metal underneath survives and a polish restores the shine.

• (a)silver chloride — Silver chloride is a real compound — the mineral horn silver — and it darkens in light, but it forms where silver meets chlorides, as in seawater or a laboratory reagent, not from the ordinary air of a room.
• (b)silver oxide — This is the tempting answer by analogy with iron rusting in air, but silver is too unreactive for oxygen to build the black film. Silver oxide is also easily broken up by heat, unlike the stable tarnish layer.
• (d)silver sulphate — A sulphate requires sulphuric acid or sulphate ions in contact with the metal, not the traces of hydrogen sulphide gas present in air. The tarnish is the sulphide, not the sulphate.

Corrosion is the slow chemical attack of a metal by its surroundings, and each metal has its own signature product. Silver goes grey-black as silver sulphide, iron goes reddish-brown as hydrated iron oxide, copper goes green as basic copper carbonate, and aluminium forms a colourless oxide skin. Tarnishing is a chemical change, because a new substance appears — which is why it cannot be wiped off like dust and why polishing removes a little silver along with the film.

The question rewards knowing that not all corrosion is oxidation by oxygen. Hydrogen sulphide is present in air in tiny amounts from natural and industrial sources, and silver is unusually sensitive to it; that is also why silver jewellery tarnishes faster in polluted or humid air and why eggs, onions and rubber bands blacken silver spoons. The contrast with rust is instructive — the Ag₂S layer is adherent and self-limiting, so it protects the silver beneath, whereas rust flakes away and exposes fresh iron, letting corrosion eat right through.

• The black tarnish on silver is silver sulphide, Ag₂S, formed from hydrogen sulphide present in the air.
• Oxygen alone attacks silver only very slowly; sulphur compounds are what produce the familiar blackening.
• Tarnishing is a chemical change — a new substance forms, and polishing takes away a thin layer of silver with it.
• The sulphide film is self-limiting and shields the silver beneath, unlike rust, which flakes and lets corrosion continue.
• Each metal corrodes to its own product — iron to reddish-brown hydrated oxide, copper to a green basic carbonate, aluminium to a protective colourless oxide.

• Answering 'oxide' by analogy with rust whenever a metal is attacked by air.
• Assuming tarnish is only a surface stain rather than a new compound.
• Mixing up sulphide (from hydrogen sulphide gas) with sulphate (from sulphuric acid or sulphate ions).

As a direct product-identification item like this, as an assertion-and-reason pair on why silver blackens, or inside a broader corrosion question pairing each metal with its corrosion product.

• practice — not a real PYQ

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

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

  Answer(b) basic copper carbonate — moist air with carbon dioxide gives copper its familiar green patina.

• practice — not a real PYQ

  Tarnishing of silver in air is

  • (a)a physical change, because the silver can be polished back
  • (b)a chemical change, because a new compound is formed on the surface
  • (c)neither a physical nor a chemical change
  • (d)a nuclear change

  Answer(b) a chemical change, because a new compound is formed on the surface — silver sulphide, which was not there before.