The diagram given below represents the industrial process for the manufacture of concentrated sulphuric acid by the Contact process .

concentrated sulphuric acid followed by water

Reason — Sulphur trioxide is not dissolved directly in water as the reaction is highly exothermic and forms a dense mist of sulphuric acid. So, SO₃ is first absorbed in concentrated sulphuric acid to form oleum (H₂S₂O₇), which is then diluted with water to obtain sulphuric acid.