Benzoic acid has a chemical formula [tex] C_{7} H_{6} O_{2} [/tex] while salicylic acid has the chemical formula [tex] C_{7} H_{6} O_{3} [/tex]. From the chemical formula alone, the differences between benzoic acid and salicylic acid is already evident as the latter has an extra oxygen in its chemical formula.
Structurally speaking, both of them are carboxylic acids with the benzene ring as the base. However, salicylic acid has an additional hydroxyl group (-OH) connected to the carbon immediately next to the carbon connected to the acidic (-COOH) group.
The structures for benzoic acid and for salicylic acid, respectively, are shown in the figures below.
