Chlorine acts as a bleaching agent only in the presence of

Which is used for sterilization of water in the water supply system of cities?

Bromine is commercially prepared from sea water by displacement reaction:
$Cl_2 + 2Br^{-} (aq.) \to 2Cl^{-}(aq.) + Br_2$
$Br_2$ gas thus formed is dissolved into a solution of $Na_2CO_3$ and then pure $Br_2$ is obtained by treatment of the solution with:

Identify the correct decreasing order of boiling point for hydrides of halogens.

When $Cl_2$ is passed through cold dil. $NaOH$,the products are:

The correct order of acidic strength is $.......?$