In the preparation of H₂SO₄ by Contact Process, why is SO₃ not absorbed directly in water to form H₂SO₄?

Dissolution of SO₃ in water is highly exothermic in nature. This leads to the formation of mist of tiny droplets which are highly corrosive and they can even attack the lead pipelines (lead pipelines are used to cover the tower in contact process).

Thus the acid fog which is formed by the direct reaction of water with sulphur trioxide is very difficult to condense. Therefore the following reaction takes place.

Absoption of SO₃ in 98 percent of H₂SO₄ to form oleum

SO₃ + H₂SO₄H₂S₂O₇

Then dilution of oleum takes place to get H₂SO_4.

H₂S₂O₇ + H₂O 2H₂SO₄