CDS (I) 2022 General Knowledge · Question 8 of 120

Correct — B, A white substance is formed when an aqueous solution of barium chloride is mixed with sodium sulphate solution. This is a double displacement reaction, not an oxidation. Barium chloride and sodium sulphate simply exchange partners — BaCl₂ + Na₂SO₄ → BaSO₄ + 2NaCl — and the white substance is barium sulphate, which is insoluble and drops out as a precipitate. Track the oxidation numbers and nothing moves: barium stays at +2 throughout, sulphur stays at +6 inside the sulphate ion, sodium stays at +1 and chlorine at −1. With no element gaining or losing electrons, there is no oxidation and no reduction. Each of the other three descriptions is a textbook case of something being oxidised, which is what leaves this one as the odd option out.

• (a)The taste of butter changes if left for a longer period. — This is rancidity, and it is oxidation. The fats and oils in butter react slowly with atmospheric oxygen to give short-chain acids and aldehydes, which is what the tongue registers as the changed taste — the reason fatty foods are packed in nitrogen and sold with antioxidants.
• (c)A reddish-brown powder gets coated on articles made of iron. — Rusting, and one of the most familiar oxidations there is. Iron goes from an oxidation state of zero in the metal to +3 in hydrated iron(III) oxide, losing electrons to oxygen in the presence of moisture.
• (d)Wine gets sour with time. — Also oxidation. Once air reaches it, the ethanol in wine is oxidised to ethanoic acid, largely by acetic-acid bacteria of the genus Acetobacter, and ethanoic acid in water is what vinegar is.

Oxidation in its widest school definition is loss of electrons, and it shows up as an increase in oxidation number; in the narrower everyday sense it is addition of oxygen or removal of hydrogen. A double displacement reaction is a different category altogether: two ionic compounds in solution swap their partners, and the driving force is usually that one of the new pairs is insoluble and leaves the solution as a precipitate. Because the ions keep their charges throughout, no electrons change hands, so a precipitation reaction is not a redox reaction at all.

The question is really asking which of four everyday changes belongs to a different class, so the fastest route is to name the class of each. Butter going off, iron rusting and wine turning sour are all slow reactions with atmospheric oxygen or with an oxidising organism. The fourth is two salt solutions being poured together, and nothing there is exposed to oxygen at all. The test that settles it beyond doubt is to assign oxidation numbers on both sides of BaCl₂ + Na₂SO₄ → BaSO₄ + 2NaCl and find that not one of them changes. Barium sulphate's insolubility is what makes the reaction visible, and it is also why a barium meal can be swallowed safely before an abdominal X-ray — the compound is so insoluble that almost no barium reaches the bloodstream.

• BaCl₂ + Na₂SO₄ → BaSO₄ + 2NaCl is a double displacement reaction, and the barium sulphate formed is an insoluble precipitate.
• No oxidation number changes in that reaction, so it is not a redox reaction.
• Rancidity is the slow oxidation of the fats and oils in food, countered by antioxidants and by nitrogen flushing.
• Rusting takes iron from an oxidation state of 0 to +3 in hydrated iron(III) oxide, and needs both air and moisture.
• Ethanol is oxidised to ethanoic acid when wine is exposed to air, largely by bacteria of the genus Acetobacter.

• Treating any reaction that produces a visible change as a redox reaction.
• Missing rancidity as oxidation because no rust or flame is involved.
• Confusing displacement, in which a more reactive element pushes out a less reactive one, with double displacement, in which two compounds exchange ions.

As an odd-one-out among everyday changes, as a classification of a written equation, or as a direct question on what the white precipitate in the barium test actually is.

• practice — not a real PYQ

  A white precipitate forms when dilute sulphuric acid is added to barium chloride solution. The precipitate is

  • (a)barium chloride
  • (b)barium sulphate
  • (c)barium hydroxide
  • (d)hydrogen chloride

  Answer(b) barium sulphate — BaCl₂ + H₂SO₄ → BaSO₄ + 2HCl, and barium sulphate is insoluble, which is why the test is used to detect sulphate ions.

• practice — not a real PYQ

  Food packets containing chips are flushed with nitrogen before sealing in order to

  • (a)keep the chips crisp by absorbing moisture
  • (b)prevent the oxidation of the fats and oils in them
  • (c)make the packet lighter to transport
  • (d)kill any bacteria present inside the packet

  Answer(b) prevent the oxidation of the fats and oils in them — nitrogen is unreactive and displaces the oxygen that would otherwise cause rancidity.