With reference to 'Silver articles', which of the following statements is/are correct ? 1. Silver articles turn black after some time when exposed to air. 2. It happens due to coating of silver sulphide. Select the correct answer from the code given below : Code :
- (a)Only 2
- (b)Both 1 and 2
- (c)Neither 1 nor 2
- (d)Only 1
Correct — B, Both 1 and 2. Statement 1 is the everyday observation: a silver ornament, coin or spoon left in the open loses its shine and goes dull grey and then black. Statement 2 gives the right reason for it. The black film is silver sulphide, Ag2S, described in the chemical literature as a dense black solid. It forms because silver reacts with the traces of hydrogen sulphide present in ordinary air — hydrogen sulphide, not oxygen, is what silver needs in order to tarnish, and plain oxygen blackens it only very slowly over long periods. The layer is also self-limiting: once a coating of silver sulphide has formed it shields the metal underneath from further attack, which is why tarnish stays a surface film and does not eat through the article. Both statements therefore stand, and both are about the same event.
- (a)Only 2 — It accepts the chemistry but rejects the observation that produced it, which is self-contradictory. If a coating of silver sulphide forms on the article, the article must change colour — silver sulphide is black, so statement 1 follows from statement 2.
- (c)Neither 1 nor 2 — Both statements are correct. Silver does tarnish on exposure to air, and the tarnish is a silver sulphide film. Ruling out both would mean silver never darkens, which contradicts ordinary experience with silverware and silver jewellery.
- (d)Only 1 — The observation is right but the explanation is wrongly rejected. This is the option for a candidate who assumes tarnishing must be oxidation and expects silver oxide. The blackening of silver is specifically a sulphide reaction — it is copper that goes green with a basic carbonate and iron that goes reddish-brown with a hydrated oxide.
Corrosion is the slow chemical attack of a metal by substances in its surroundings, and it produces a different-coloured compound on the surface in each case. Iron rusts to a flaky reddish-brown hydrated oxide, copper acquires a green coat of basic copper carbonate, aluminium forms a thin colourless film of aluminium oxide that protects the metal beneath, and silver blackens with silver sulphide. Silver is the odd one out because the culprit is not oxygen but sulphur: the small quantities of hydrogen sulphide that occur in air are enough to convert the bright surface into Ag2S, a compound that also occurs in nature as the minerals acanthite and argentite.
The question is really asking whether you know that not all corrosion is oxidation. Because 'reacts with air' is in the stem, the reflex is to reach for oxygen and therefore for silver oxide, which makes option (d) attractive. The safe way to hold this is by colour: black means sulphide and means silver, green means carbonate and means copper, brown-red means oxide and means iron. The same reasoning explains a familiar kitchen observation — a silver spoon used with sulphur-rich food such as egg yolk darkens far faster than one left in a drawer, because the sulphur supply is what limits the reaction. Since tarnishing produces a new substance it is a chemical change, not a physical one, and it can be reversed chemically rather than by simply wiping.
- The black tarnish on silver is silver sulphide, Ag2S — a dense black solid
- It forms from the traces of hydrogen sulphide in the air; silver needs hydrogen sulphide to tarnish, and tarnishes with oxygen alone only very slowly
- The sulphide layer is protective — once formed, it shields the silver underneath from further conversion
- Silver sulphide occurs naturally as the minerals acanthite (the low-temperature monoclinic form) and argentite (the high-temperature cubic form)
- Colour code for corrosion: silver goes black (sulphide), copper goes green (basic copper carbonate), iron goes reddish-brown (hydrated iron oxide), aluminium forms a colourless protective oxide film
- Tarnishing is a chemical change because a new substance is formed
- Assuming the black coating is silver oxide because the stem says 'exposed to air' — the tarnish is a sulphide
- Mixing up the corrosion colours: black for silver, green for copper, reddish-brown for iron
- Treating tarnishing as a physical change or as mere dirt; a new compound is formed, so it is a chemical change
- Believing tarnish eats through the article — the sulphide film is protective and stays on the surface
UPPSC puts this as a two-statement observation-plus-reason item, testing whether the everyday fact and its chemistry are held together; UPSC has asked the surrounding chemistry instead — which surface layer a treatment deposits on a metal, and whether a given change counts as chemical or physical.
Aluminium surfaces are often “anodized”. This means the deposition of a layer of
- (a) chromium oxide
- (b) aluminium oxide
- (c) nickel oxide
- (d) zinc oxide
Answer(b) aluminium oxide
The same idea from the other direction — a surface layer of a compound sitting on a metal and changing what the metal looks like and how it behaves. Anodising deliberately grows a protective aluminium oxide film; tarnishing grows a silver sulphide film by itself. Both are surface compounds, and neither is the metal.
Which of the following is/are the example/examples of chemical change? 1. Crystallization of sodium chloride 2. Melting of ice 3. Souring of milk Select the correct answer using the code given below.
- (a) 1 and 2 only
- (b) 3 only
- (c) 1, 2 and 3
- (d) None
Answer(b) 3 only
The classification this question quietly depends on. A chemical change is one in which a new substance appears; the blackening of silver qualifies because silver sulphide is a new compound, exactly as souring of milk qualifies there while melting and crystallisation do not.
- practice — not a real PYQ
A copper vessel left exposed to moist air develops a green coating. This green substance is
- (a)copper sulphide
- (b)basic copper carbonate
- (c)copper nitrate
- (d)copper chloride
Answer(b) basic copper carbonate — copper goes green with a carbonate, whereas silver goes black with a sulphide.
- practice — not a real PYQ
Which one of the following statements about the tarnishing of silver is correct?
- (a)It is a physical change and the metal can be restored by wiping
- (b)It is caused mainly by nitrogen in the air
- (c)It is a chemical change in which silver sulphide is formed
- (d)It is caused by carbon dioxide dissolving in moisture on the surface
Answer(c) It is a chemical change in which silver sulphide is formed — a new compound, Ag2S, appears on the surface.