Which one of the following gas smells like a rotten egg?

Correct — B, Hydrogen sulphide. The rotten-egg smell is hydrogen sulphide's identifying property, and the comparison is not accidental: an egg white is rich in sulphur-containing amino acids, and when it spoils, bacteria release that sulphur as H₂S. The same gas rises from marshes, sewers, well water and volcanic vents, wherever sulphate-reducing bacteria or hot rock are at work, and it is what makes a rotten-egg smell a reliable warning of anaerobic decay. Two further things about it matter more than the smell. It is genuinely poisonous, blocking cellular respiration in much the same way as hydrogen cyanide. And it destroys its own warning: very pungent at first, it quickly deadens the sense of smell, so that at dangerous concentrations a person may believe the gas has cleared when in fact only their nose has stopped reporting it. Of the four options, only hydrogen sulphide and sulphur dioxide contain sulphur at all, and only one of the two is the rotten-egg gas.

• (a)Ammonia — Ammonia, NH₃, has a sharp, pungent, eye-watering smell — the smell of a household cleaning fluid, quite unlike rotten eggs. It contains no sulphur, and its role in the syllabus is as the feedstock for fertilizers and as a refrigerant.
• (c)Acetylene — Acetylene, C₂H₂, is the fuel burned in an oxy-acetylene welding torch and is also used to ripen fruit. It contains no sulphur at all, and a rotten-egg smell needs sulphur to produce it.
• (d)Sulphur dioxide — The tempting wrong answer, because sulphur dioxide really does contain sulphur. Its smell is different in kind, though — the sharp, choking odour of a freshly struck match or of burning sulphur. SO₂ is the gas of acid rain and of volcanic plumes.

Hydrogen sulphide, H₂S, is a colourless gas produced whenever sulphur-containing organic matter decays without oxygen, and also released by volcanoes and hot springs. It is denser than air, so it collects in pits, drains and tanks, which is why it kills sewer and tanker workers. In the school laboratory it is made by the action of dilute acid on iron sulphide, and it is the classic reagent of qualitative analysis, precipitating metal sulphides of characteristic colours. Its toxicity works by inhibiting cellular respiration in a manner similar to hydrogen cyanide.

Smell questions look like pure recall, but they can be reasoned. Start by asking which options could produce a sulphurous smell at all: ammonia is a nitrogen compound and acetylene is a hydrocarbon, so neither is a candidate. That leaves hydrogen sulphide and sulphur dioxide, and the two are easy to separate once you have attached each to a familiar experience — rotten eggs and stagnant drains for the first, a struck matchstick and a burning heap of sulphur for the second. The practical lesson buried in the item is worth remembering beyond the exam: because H₂S paralyses the sense of smell within a few breaths at high concentration, the disappearance of the smell is a danger signal rather than an all-clear.

• Hydrogen sulphide, H₂S, is the gas with the characteristic foul odour of rotten eggs.
• It is produced by sulphate-reducing bacteria in swamps, sewers and well water, and is emitted by volcanoes and hot springs.
• It is toxic to humans, inhibiting cellular respiration in a manner similar to hydrogen cyanide.
• At concentrations of roughly 100 to 150 parts per million the olfactory nerve is paralysed within a few breaths and the smell vanishes, though the danger does not.
• Sulphur dioxide, the other sulphur gas on the list, smells instead of a struck match and is the principal cause of acid rain.

• Picking sulphur dioxide because it obviously contains sulphur; its smell is the burnt-match one.
• Assuming a smell that vanishes means the gas has gone — with hydrogen sulphide it usually means the nose has stopped working.
• Confusing hydrogen sulphide with the biogas methane; both come from anaerobic decay, but methane is odourless.

Usually as a one-line identification of a gas by smell, or as a match between gases and their sources, or as a statement item on which gas is toxic or which causes acid rain.

• practice — not a real PYQ

  The gas responsible for the smell of a rotten egg is produced in swamps and sewers mainly by

  • (a)photosynthetic algae
  • (b)sulphate-reducing bacteria acting without oxygen
  • (c)evaporation of sea water
  • (d)the action of sunlight on ozone

  Answer(b) sulphate-reducing bacteria acting without oxygen — anaerobic decay of sulphur-containing matter is the usual source of hydrogen sulphide in swamps, sewers and well water.

• practice — not a real PYQ

  Which one of the following gases is the principal cause of acid rain?

  • (a)Hydrogen sulphide
  • (b)Sulphur dioxide
  • (c)Acetylene
  • (d)Helium

  Answer(b) Sulphur dioxide — released chiefly by burning sulphur-bearing fuels, it is oxidised in the atmosphere and dissolves to give acid rain. Hydrogen sulphide is the rotten-egg gas, a different sulphur compound.