Q.:- Why does dry HCl gas not change the colour of the dry litmus paper?
We know that the colour of the litmus is changed by H+ ions of an acid. Dry HCl does not dissociate to give H+ ions. It is only in the aqueous that an acid dissociate to give ions. Since in this case, neither HCL is in aqueous form nor is the litmus paper wet, so the colour of litmus paper does not change.