Question

Why does dry HC1 gas not change the colour of the dry litmus paper?

Answer 1

Because it does not contain (H+) hydrogen ions

Answer 2

The colour of the litmus paper changes only when there is a presence of hydrogen or hydronium ion. (H^3O). HCl produces these ions only in the form of aqueous solution .

So dry HCl does not change the colour of litmus paper