The best method for the separation of naphthalene and benzoic acid from their mixture is::

Naphthalene can sublime directly to vapour upon heating, but benzoic acid do not. Related Theory When benzoic acid is heated in presence of $a$ strong dehydrating agent like $\mathrm{P}_2 \mathrm{O}_5$ or $\mathrm{H}_2 \mathrm{SO}_f$ it forms benzoic anhydride. $$ 2 \mathrm{C}_6 \mathrm{H}_5 \mathrm{COOH} \underset{\mathrm{BaOl}_5}{\longrightarrow}\left(\mathrm{C}_6 \mathrm{H}_5 \mathrm{CO}\right)_2 \mathrm{O}+\mathrm{H}_2 \mathrm{O} $$ Caution Naphthalene is volatile and benzoic acid is non-volatile due to the formation of dimer via hydrogen bonding (intermolecular).