Question

Moist $S{O}_2$ behaves as a __ agent.

Answer 1

When moist, $S{O}_2$ behaves as a reducing agent. It reduces $F{e}^{3+}$ to $F{e}^{2+}$ as
$2F{e}^{3+}S{O}_2+H_{2}O\to 2F{e}^{2+}+4H^{+}+S{O}_4^{2-}$
A convenient test for the gas is the decolourization of acidified potassium permanganate solution:
$5S{O}_2+2Mn{O}_4^{-}+2H_{2}O\to 5S{O}_4^{2-}+4H^{+}+2M{n}^{2+}$
Do note that $S{O}_2$ here is taken within the context of being a reducing agent in moist conditions. This does not imply that $S{O}_2$ can only be a reducing agent as can be confirmed by the reaction following reaction:
$2H_{2}S+S{O}_2\to 3S+2H_{2}O$
Similarly, $S{O}_2$ can oxidize active metals like Magnesium to MgO and MgS. With potassium $K_2{S}_2{O}_3$ and $K_{2}S{O}_3$ are formed.