Question

Why is dry HCl gas not turned blue litmus red while it turns moist blue litmus red?​

Answer 1

For blue litmus paper to turn red, hydronium(H30+) ions are required, these are formed when H+ ions react with water(H+ + H2O⇌H3O+).

Dry HCl gas does not undergo ionization on touching dry blue litmus paper, ionization takes place to form H3O+ and Cl- ions only in the presence of water(polar covalent with high dielectric constant), hence moist blue litmus turns red on exposure to HCl gas while dry blue litmus remains blue.