Question

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

Answer 1

Dry HCl gas does not change the colour of dry litmus paper because it has no hydrogen ions $\left({\text{H}}^{+}\right)$ in it. Only in aqueous solution, acids can dissociate into ions. Since HCl is not aqueous and the litmus paper is not wet in this case, the colour of the litmus paper does not change. So dry hydrogen chloride gas has no effect on litmus paper but when hydrogen chloride gas is passed through moist blue litmus paper, the litmus paper changes to red.