CDS (II) 2021 General Knowledge · Question 10 of 120

Correct — A, Cupric carbonate. Copper left in damp air slowly loses its shiny brown surface and takes on a green coat, and the textbook names the culprit reaction directly: copper reacts with moist carbon dioxide in the air, and the green substance formed is basic copper carbonate. Of the four options only cupric carbonate — copper in its plus-two state, paired with the carbonate ion — belongs to that family, which is why the key takes it. The other three do not fit the description in the stem. Cuprous oxide is red, cupric oxide is black, and copper sulphate needs a sulphur source that the stem never supplies; carbon dioxide and moisture cannot deliver sulphate. The corrosion is worth distinguishing from rusting: iron rust flakes away and leaves fresh metal exposed to attack, whereas the green copper layer is adherent and seals the surface underneath, which is why copper roofs and statues survive for centuries once they have turned green.

• (b)Cuprous oxide — Copper(I) oxide is red, not green, and forming it needs oxygen rather than carbon dioxide. It is a high-temperature product, not the coat that grows on copper in damp air.
• (c)Cupric oxide — Copper(II) oxide is black. Heating copper in air produces it, which is a different experiment from leaving copper in moist carbon-dioxide-bearing air.
• (d)Copper sulphate — There is no sulphur in the reactants the stem names. Hydrated copper sulphate is indeed blue, and that colour is what makes this option tempting, but moist carbon dioxide cannot produce a sulphate.

Corrosion is the slow attack of a metal by its surroundings — air, moisture, acids and dissolved gases. Silver exposed to air reacts with sulphur compounds in it and blackens with silver sulphide; iron in moist air acquires a brown flaky rust; copper in moist air bearing carbon dioxide develops a green coat of basic copper carbonate. All three are chemical changes, not the physical deposit of dirt, and each involves a specific reactant besides oxygen.

The item can be settled purely on colour and on the elements available. Green rules out the two oxides straight away, since one is red and the other black, and the reactants named in the stem rule out any sulphate. One point of vocabulary deserves an honest note. The textbook calls the product basic copper carbonate, meaning the mixed hydroxide-carbonate of formula Cu2(OH)2CO3 — the mineral malachite — and not the plain neutral salt CuCO3, which is in fact difficult to prepare in pure form. The paper's option prints 'cupric carbonate', which is the closest of the four to the textbook's name and the only copper(II) carbonate on offer, and the key marks it correct on that basis. Outdoors the chemistry is more mixed still: patina on copper roofs in polluted or coastal air is often dominated by basic copper sulphate or chloride rather than the carbonate.

• Copper reacts with moist carbon dioxide in the air, loses its shiny brown surface and gains a green coat of basic copper carbonate.
• Silver articles turn black in air because silver reacts with sulphur compounds to form a coating of silver sulphide.
• Iron exposed to moist air for a long time acquires a brown flaky coating of rust; both water and oxygen are needed.
• Copper(I) oxide is red and copper(II) oxide is black, so neither can be the green coat described in the stem.
• The green layer on copper is adherent and protects the metal beneath, unlike rust on iron, which flakes off and exposes fresh metal.

• Reaching for copper sulphate because its hydrated form is a familiar blue crystal; the stem's reactants contain no sulphur.
• Mixing up the colours of the two copper oxides — cuprous is red, cupric is black.
• Assuming every corrosion product weakens the metal; the green copper layer protects the surface beneath it.

As a name-the-green-coat item like this one, as a match-list pairing metals with the colour of their corrosion product, or as a statements question on how rusting differs from the tarnishing of silver and copper.

• practice — not a real PYQ

  A silver ornament turns black when left exposed to air for a long time. The black substance is

  • (a)silver oxide
  • (b)silver carbonate
  • (c)silver sulphide
  • (d)silver chloride

  Answer(c) silver sulphide — silver reacts with sulphur compounds present in air to form a black coating.

• practice — not a real PYQ

  Which two conditions together are necessary for iron to rust?

  • (a)Air and sunlight
  • (b)Water and oxygen
  • (c)Carbon dioxide and heat
  • (d)Nitrogen and moisture

  Answer(b) Water and oxygen — iron kept in dry air or in boiled water free of dissolved oxygen does not rust.