Why does dry HCl gas not change the colour of the dry litmus paper?

Explanation

Colour of the litmus paper is changed by the hydrogen ions. Dry HCl gas does not contain `H^(+)` ions. It is only in the aqueous solution that an acid dissociates to give ions. Since in this case, neither HCl is in the aqueous form nor the litmus paper is wet, therefore, the colour of the litmus paper does not change.