Chlorine acts as a bleaching agent only in the presence of moisture - explain.

Chlorine is a powerful bleaching agent. bleaching action is due to oxidation in the presence of water, to liberate nascent oxygen.
${\mathrm{Cl}}_2+{\mathrm{H}}_2\mathrm{O}\to 2\mathrm{HCl}+\left(\mathrm{O}\right)$
Coloured substance $+\left(\mathrm{O}\right)\to$ colourless