Silver articles turn black when kept in the open for longer time due to the formation of
- (a)H₂S
- (b)AgS
- (c)AgSO₄
- (d)Ag₂S
Correct — D, Ag₂S. Silver does not rust and does not react with the oxygen of the air, but it does react with sulphur and with sulphur compounds, and ordinary air carries traces of hydrogen sulphide. The two combine to give silver sulphide, a dense black solid, and it is the thin film of Ag₂S that dulls a silver spoon or an idol left out for months. The formula follows straight from valency: silver is univalent and sulphide is divalent, so two silver atoms are needed for each sulphur and the compound must be Ag₂S. This is tarnishing, a form of corrosion and a chemical change, since a new substance has been produced. The coating is self-limiting — once formed, it shields the metal underneath — which is why an old silver article goes black but does not crumble away.
- (a)H₂S — Hydrogen sulphide is the cause, not the product. It is the trace gas in the air that supplies the sulphur, but it is itself a colourless gas and cannot be the black film on the metal.
- (b)AgS — A valency error made to look plausible. Silver forms the Ag⁺ ion and sulphur the S²⁻ ion, so the charges balance only at two silver to one sulphur. AgS is not a compound the chemistry allows.
- (c)AgSO₄ — Wrong on two counts. The sulphate ion is SO₄²⁻, so with univalent silver the formula would have to be Ag₂SO₄, and in any case silver sulphate is a white, not a black, solid. Tarnish is a sulphide, not a sulphate.
Corrosion is the slow chemical attack of the surface of a metal by the substances around it, and each metal blackens, greens or reddens in its own way. Iron in moist air gives reddish-brown hydrated iron oxide, rust, which flakes and exposes fresh metal. Copper in moist air develops a green coating of basic copper carbonate. Aluminium forms a thin colourless oxide layer that protects it. Silver gives black silver sulphide. The pattern to carry is that the coloured coating names the reaction.
Two independent lines lead to the same answer, and using both is safer than using either. First, chemistry: silver's characteristic reaction in air is with sulphur compounds, not with oxygen, so the product is a sulphide and options mentioning sulphate or the gas itself are out. Second, formula-writing: silver is univalent and sulphide divalent, so the only well-formed sulphide of silver among the options is Ag₂S. Even a student who has forgotten the chemistry can eliminate AgS and AgSO₄ purely on valency. The compound occurs naturally as the minerals acanthite and argentite.
- Silver tarnishes to black silver sulphide, Ag₂S, by reaction with traces of hydrogen sulphide in the air.
- Silver does not react with the oxygen of the air even on strong heating, which is why the tarnish is a sulphide and not an oxide.
- The formula follows from valency: Ag⁺ with S²⁻ requires two silver atoms per sulphur.
- Silver sulphide occurs as the minerals acanthite and argentite, and the tarnish film protects the metal beneath it.
- Compare the other corrosion products: iron gives reddish-brown hydrated iron oxide, copper a green basic carbonate, aluminium a colourless protective oxide.
- Assuming that anything blackening in air must be oxidising — silver's tarnish is a sulphide.
- Writing AgS or AgSO₄ by forgetting that silver is univalent and sulphate divalent.
- Calling tarnishing a physical change; a new compound is formed, so it is chemical.
As a formula-identification item, as a 'why does silver blacken' reasoning item, or inside a corrosion set matching each metal to its coating.
Silver artefacts get tarnished in air due to the formation of
- (a) silver chloride
- (b) silver oxide
- (c) silver sulphide
- (d) silver sulphate
Answer(c) silver sulphide
The same tarnishing reaction named rather than written as a formula, with silver oxide and silver sulphate set as the traps in both papers.
CDS_GK_2020_I_Q232020Silver 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
Answer(c) silver reacts with sulphur in the air to form a coating of silver sulphide
The same fact asked in the earlier session of the same year, in words rather than in a formula. Together the two items show the examiner testing first the reaction and then the ability to write its product correctly as 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 oxide
- (d)copper chloride
Answer(b) basic copper carbonate — formed by the action of moist air containing carbon dioxide on the metal surface.
- practice — not a real PYQ
Tarnishing of silver in air is best classified as
- (a)a physical change
- (b)a chemical change
- (c)sublimation
- (d)a nuclear change
Answer(b) a chemical change — a new substance, silver sulphide, is formed, and the original metal cannot be recovered by any physical means.