Give reasons for the following:
$(a)$ Dry $HCl$ gas does not change the colour of dry litmus paper.

(N/A) The acidic character of a substance is due to the presence of $H^{+}$ ions.
In the case of dry $HCl$ gas,there is no water present to facilitate the dissociation of $HCl$ molecules into $H^{+}$ and $Cl^{-}$ ions.
Since $H^{+}$ ions are responsible for the acidic property of changing the colour of litmus paper,the absence of these ions in dry $HCl$ means the litmus paper will not change colour.