Sulphuric acid is manufactured by the contact process.

It involves the following steps:

Step (i): Sulphur or sulphide ores are burnt in air to form SO₂.

Step (ii): By a reaction with oxygen, SO₂ is converted into SO₃ in the presence of V₂O₅ as a catalyst.

2SO₂(g) + O₂(g) \(\overset{V_2O_5}\longrightarrow\) 2SO₃(g)

Step (iii): SO₃ produced is absorbed on H₂SO₄ to give H₂S₂O₇(oleum).

SO₃ + H₂SO₄ → H₂S₂O₇

This oleum is then diluted to obtain H₂SO₄ of the desired concentration.

In practice, the plant is operated at 2 bar (pressure) and 720 K (temperature). The sulphuric acid thus obtained is 96- 98% pure.