Dry HCl acts as a gas - hydrogen chloride. It does not act as an acid until present in an aqueous medium, that is, an acid can show acidic behaviour only in the presence of Protons (H+ ions). In the presence of an aqueous medium, the acid dissociates into H+ ions which contributes to its acidic behaviour. The colour change of a litmus paper from blue to red is the characteristic of acids but in the absence of water it will not show the acidic properties. An acid requires water to show its properties.