Question

Which substance/ion is oxidised and which substance/ion is reduced in the following reaction?
$C{l}_2+S{O}_3^{2-}+H_{2}O\to 2C{l}^{-}+S{O}_4^{2-}+2H^{+}$.

Answer 1

$C{l}_2+S{O}_3^{2-}+H_{2}O\to 2C{l}^{-}+S{O}_4^{2-}+2H^{+}$.
Oxidised- $S{O}_3^{2-}$ The oxidation number of S increases from +4 to +6. Thus, S itself is oxidised and acts as reducing agent.
Reduced- $C{l}_2$. The oxidation number of Cl decreases from 0 to -1. Thus, Cl itself is reduced and acts as oxidising agent.

Note:
Any species that is oxidised acts as reducing agent.

Any species that is reduced acts as oxidising agent.

Gain of oxygen (or loss of hydrogen) is oxidation.