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

Hydrogen chloride gas (HCl) does not change the colour of dry litmus paper because hydrogen chloride gas does not contain any hydrogen ions $\left({\text{H}}^{+}\right)$ in it. When HCl gas is dissolved in water, it results in the formation of an aqueous solution of HCl which contains ${\text{H}}^{+}$ and ${\text{Cl}}^{-}$ ions. When a few drops of this solution are poured on blue litmus paper, we can observe its colour changing to red.

Similarly, when hydrogen chloride gas is passed through moist blue litmus paper, the litmus paper changes to red.
[2 Marks]