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

Dry HCl gas does not change the color of dry litmus paper because it does not contain free hydrogen ions (H+ or H3O+ ions) in its dry form. The acidic property of HCl, which causes litmus paper to change color, depends on the presence of these hydrogen ions. However, dry HCl gas cannot dissociate to produce hydrogen ions unless it is dissolved in water (in an aqueous solution). Since dry litmus paper lacks moisture, the HCl gas does not produce hydrogen ions to cause the color change. Therefore, dry HCl gas does not show acidic behavior or change the color of dry litmus paper.