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.

(A) The solid $'A'$ is sulphur $(S_8)$ and the gas $'B'$ is sulphur dioxide $(SO_2)$.
$1$. Combustion of sulphur in air:
$S_8(s) + 8O_2(g) \rightarrow 8SO_2(g)$
$2$. Reaction with lime water:
$SO_2(g) + Ca(OH)_2(aq) \rightarrow CaSO_3(s) + H_2O(l)$
(Calcium sulphite makes the solution milky)
$3$. Decolourisation of acidified $KMnO_4$:
$5SO_2 + 2MnO_4^{-} + 2H_2O \rightarrow 5SO_4^{2-} + 4H^{+} + 2Mn^{2+}$
$4$. Reduction of $Fe^{3+}$ to $Fe^{2+}$:
$2Fe^{3+} + SO_2 + 2H_2O \rightarrow 2Fe^{2+} + SO_4^{2-} + 4H^{+}$