Sulphonation is a reversible reaction. When heated with excess of conc. `H_2SO_4`, the reaction moves in the forward direction giving benzenesulphonic acid and `H_2O`. However, when benzenesulphonic acid is heated with excess of steam , the reaction moves in the backward direction giving benzene and `H_2SO_4`.
`underset"Benzene"(C_6H_6)+H_2SO_4 overset"Heat"to underset"Benzenesulphonic acid"(C_6H_5SO_3H)+H_2O`
`underset"Benzenesulphonic acid"(C_6H_5SO_3H)+underset"Steam"(H_2O) overset"Heat"to underset"Benzene"(C_6H_6)+ H_2SO_4`
In contrast, nitration is an irreversible reaction. Therefore, when nitrobenzene is heated with steam, backward reaction does not occur and hence nitrobenzene is recovered .
`underset"Benzene"(C_6H_6)+underset"Conc."(HNO_3) overset(Conc. H_2SO_4)to underset"Nitrobenzene"(C_6H_5NO_2)+H_2O`
