An amorphous solid "A" burns in air to form a gas "B" which turns lime water milky. The gas is also produced as a by-product during roasting of sulphide ore. This gas decolourises acidified aqueous `KMnO_4` solution and reduces `Fe^(3+)` to `Fe^(2+)` Identify the solid "A" and the gas "B" and write the reactions involved.

Explanation

(A) `S_(8)`, (B) `SO_(2)`
with oxygen (air), `S_(8)` oxidises to `SO_(2)`
`S_(8) + 8O_(2) overset(Delta)to 8SO_(2)`
`SO_(2) + Ca(OH)_(2) to CaSO_(3) + H_(2)O`
`5SO_(2) + underset("violet")(2MnO_(4)^(-)) + 2H_(2)O to 5SO_(4)^(2-) + 4H^(+) + underset("Colourless") (2Mn^(2+))`
`2Fe^(3+) + SO_(2) + 2H_(2)O to underset("green")(2Fe^(2+)) + SO_(4)^(2-) + 4H^(+)`