Question

Give a reason why anhydrous hydrogen chloride gas and anhydrous liquified hydrogen chloride are not acids

Answer 1

This question does not makes much sense as both pure hydrogen chloride and its aqueous solution depends on the same principle for their acidic properties.
It is like asking which one is more nutritious - a raw banana or the ripe one!
Pure, anhydrous HCl is a covalent compound without any significant presence of ions. But aqueous HCl is hydrochloric acid in which the HCl molecules are completely dissociated into hydrogen (H+) and chloride (Cl-) ions. They are different only in form, not in the active component.
But, kinetically, aqueous HCl or hydrochloric acid is more reactive than pure HCl gas. Thus, hydrochloric acid readily displaces hydrogen from active metals like iron at room temperature.
Fe(s) + 2HCl (aq) = FeCl2 (aq) + H2 (g)
Dry or anhydrous HCl gas also reacts with such metals in similar fashion, but it requires high temperatures to react effectively. When the dry gas is passed over heated iron, anhydrous iron(II) chloride is produced along with hydrogen gas.
Fe (s) + 2HCl (g) = FeCl2 (s) + H2 (g)
High temperatures are required with dry HCl gas because higher activation energy is needed for breaking the covalent H-Cl bond during the course of the reaction. In aqueous HCl (hydrochloric acid), the H-Cl bond is already broken by interaction with water molecules.
Similarly, hydrochloric acid readily neutralises basic substances like pure, dry CaO or CaCO3 at room temperature, but dry or anhydrous HCl gas should need high temperatures for such neutralisation reactions. Again, the reason is the requirement of higher activation energy for dry HCl gas to react than for aqueous HCl.