Dry hydrogen chloride gas does not turn blue litmus paper red whereas hydrochloric acid does. Why?

Dry hydrogen chloride gas (HCl) does not turn blue litmus paper red because it does not contain any water molecules to dissociate into hydrogen ions (H⁺) and chloride ions (Cl⁻). The characteristic acidic properties of hydrochloric acid (HCl) result from its dissociation in water, producing H⁺ ions. These H⁺ ions are responsible for the acidity of the solution and the characteristic reaction with litmus paper, turning it red.

When HCl gas is dissolved in water, it forms hydrochloric acid, HCl(aq), which ionizes into H⁺ and Cl⁻ ions:

HCl(g) + H2O(l) → H3O⁺(aq) + Cl⁻(aq)

The H₃O⁺ ions formed by the dissociation of HCl in water are what cause the acidity in the solution and turn blue litmus paper red. However, in the absence of water, as in the case of dry hydrogen chloride gas, there are no H₃O⁺ ions produced, so the litmus paper does not undergo the characteristic color change to red.