Question

why does the colour of blue Litmus Paper not change when we pass it through dry HCL​

Answer 1

Dry HCL it does not dissociate to give H+ ions. Acids dissociate to give ions only in the aqueous solution. In this HCL is not in aqueous form nor litmus paper is wet, so the colour of litmus paper does not change.

Answer 2

Answer:

This is because the HCl gas is not acidic in nature but when it combines with water it produces hydronium ions which cause it turn wet blue litmis paper to blue.