Question

why does dry HCL gas not change the colour of dry Litmus Paper?​

Answer 1

Dry HCL has no reaction because it is not in molten state . But dry HCL can show response on wet litmus paper.

Answer 2

Dry HCl does not contain any hydrogen ions in it. So it does not show any acidic behaviour.

HCl gas can produce these ions only in the form of aqueous solution.