(i) Olfactory indicator: A substance whose odour changes in acidic or basic media, e.g., onion, vanilla essence, clove oil.
(ii) Water of crystallisation: The fixed number of water molecules chemically bonded in the crystalline structure of a salt, e.g., CuSO₄·5H₂O.
Source: Chapter 2, Section 2.1.1
The question asks for two definitions but is worth 1 mark total — keep each definition to one short phrase. Olfactory indicator is directly defined in the passage. Water of crystallisation is a standard textbook definition from Chapter 2 (not in the given passages, but is a core term from the same chapter). Examiners expect the definition plus one example for each.