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Roasting of a sulphide ore in excess air always evolves a colourless gas whose aqueous solution is acidic. The gas is
A$SO_2$
B$CO_2$
C$H_2S$
D$NO_2$
Answer & Solution
Correct answer: A. $SO_2$
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_
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