why does dry hcl gas not change the colour of dry litmus paper

Dry HCl gas does not change the colour of dry litmus paper because it does not contain free hydrogen ions (H+). The colour change in litmus paper occurs only in the presence of these hydrogen ions or hydronium ions (H3O+), which are produced when HCl dissolves in water. Since dry HCl gas is not in aqueous form and the litmus paper is dry (lacking moisture), the HCl gas cannot dissociate to release H+ ions, and therefore no acid-base reaction occurs to change the litmus paper colour. Essentially, dry HCl gas is neutral in this context due to the absence of water that facilitates ionization.