Question

dry HCL gas does not affect the dry strip of blue Litmus Paper but it turns red in the presence of a drop of water

Answer 1

acids show acidic character only in acidic medium

Answer 2

Hola...!!

HCl shows its acidic characters in the presence of water. As the H+ ions get ionised in the presence of water.
That's why, the dry HCl gas does not affect the dry strip of blue litmus paper, but it turns red litmus blue in the presence of a drop of water.

Hope it'll help....