Dry $HCl$ gas does not change the colour of dry blue litmus. Give reason.

(N/A) The acidic character of $HCl$ is due to the presence of $H^+$ ions.
In the absence of water,$HCl$ molecules do not dissociate to produce $H^+$ ions.
Since the litmus paper is dry and the $HCl$ gas is also dry,there is no medium for the dissociation of $HCl$ into ions.
Therefore,the litmus paper does not change its colour.