Write a note on preparation of sulphur dioxide.

Explanation

Sulphur dioxide is formed together with a little (6-8 %) sulphur trioxide when sulphur is burnt in air or oxygen :
`S_(s) + O_(2)(g) to SO_(2)(g)`
In the laboratory it is readily generated by treating a sulphite with dilute sulphuric acid.
`SO_(3)^(2-) (aq) + 2H_(aq)^(+) to H_(2)O(l) + SO_(2)(g)`
Industrially, the `SO_2` is obtained as a by-product of the roasting of sulphide ores.
`4FeS_(2)(s) + 11O_(2)(g) to 2Fe_(2)O_(3)(g) + SO_(2)(g)`
The gas after drying is liquefied and stored in steel cylinders.