Roasting of a sulphide ore in excess air always evolves a colourless gas whose aqueous solution is acidic. The gas is

1. Roasting oxidises the sulphide, e.g. $2ZnS + 3O_2 \rightarrow 2ZnO + 2SO_2$. 2. The evolved gas is $SO_2$, colourless, with a pungent smell. 3. $SO_2$ dissolves in water to give sulphurous acid $H_2SO_3$, so the solution is acidic. _Source: Samacheer Kalvi Class 12 Chemistry Vol.1, Unit 1 'Metallurgy', §1.3.1 / Evaluation Q.2_