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

The correct answer is B, Hydrogen sulphide. H₂S is a colourless gas with the familiar smell of rotten eggs, and the human nose can detect it even in very small amounts.

Hydrogen sulphide forms when bacteria break down organic matter containing sulphur in the absence of oxygen, so it is found in sewers, swamps, volcanic gases and rotting eggs, whose proteins are rich in sulphur. It is poisonous: at high concentration it dulls the sense of smell, so a worker may stop noticing it just when it is most dangerous. In the laboratory it is made by treating iron sulphide with dilute acid and is used to test for metal ions.

A, ammonia, has a sharp, pungent smell that stings the eyes, as near a urinal. C, acetylene, is nearly odourless when pure; the garlic-like smell of welding gas comes from impurities. D, sulphur dioxide, has the choking smell of a burnt matchstick.

Exam tip: rotten egg is H₂S, burnt match is SO₂, pungent is NH₃.