Sulphur is burnt in air to produce sulphur dioxide.
Sulphur dioxide that is formed is mixed with excess oxygen and is conducted through the catalyst vanadium (V) oxide to raise the rate of reaction. A temperature of 500°C and a pressure of l to 2 atmospheres is used.
Sulphur trioxide that is formed is dissolved in concentrated sulphuric acid to form oleum at stage two.
Sulphur trioxide does not dissolve directly in the water because this reaction releases too much heat and could even produce an explosion. Then,,, water is mixed with the oleum to produce concentrated sulphuric acid.