Nitrobenzene can be prepared from benzene by using a mixture of conc. ${\text{HNO}}_3$ and conc. ${\text{H}}_2{\text{SO}}_4$. In this mixture, nitric acid acts as a/an:

In the nitrating mixture:${\text{HNO}}_3+2{\text{H}}_2{\text{SO}}_4\to {\text{NO}}_2^{+}+{\text{H}}_3{\text{O}}^{+}+2{\text{HSO}}_4^{-}$ Here, sulfuric acid is a stronger acid than nitric acid. It protonates nitric acid, which then loses water to form the nitronium ion. By accepting a proton, nitric acid acts as a base.