= Salicylic acid
<Salicylic acid> is 2-hydroxybenzoic acid, $\mathrm{C_7H_6O_3}$: a <benzene> ring bears adjacent hydroxyl and <carboxylic acid> groups.<NIST Chemistry WebBook, SRD 69>[Salicylic acid, CAS69-72-7: formula and 2-hydroxybenzoic-acid structure]{ref}
The two groups supply distinct chemical sites. Acid–base reactions can change their protonation, while derivatization at the phenolic oxygen changes the molecular identity. Their adjacent arrangement also permits intramolecular <hydrogen bonding>, so moving one group to a different ring position can alter properties despite preserving elemental composition. This makes the compound a useful example of the difference between a formula and a positional isomer.
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