Final answer:
Acid-base reactions are most common in aqueous solutions but can also occur in non-aqueous environments like in pure or molten states of acids and bases. These reactions involve the transfer of protons, which can happen outside of aqueous solutions as per broader definitions such as those by Bronsted-Lowry and Lewis.
Step-by-step explanation:
Acid-base reactions typically occur in aqueous solutions because water's unique properties provide a medium for ions to move and react. However, these reactions can also take place in non-aqueous environments, such as in pure forms or molten states of acids and bases, although these circumstances are less common. For instance, when acids and bases are in their molten state, they can still donate and accept protons, which is the hallmark of an acid-base reaction. In such a scenario, the absence of water forces the acid and base to interact directly with each other, producing the corresponding salt and possibly water if the acid is a hydracid (an acid that releases hydrogen ions when dissolved).
Acid-base reactions in their broadest definition are not confined to aqueous solutions only. Even the Arrhenius definition, which is limited to aqueous solutions, hints at the possibility of broader applications. A reaction where hydrogen chloride (HCl) and water react to form hydronium ions (H3O+) and chloride ions (Cl−) is an example of an acid reacting in water. However, other definitions, like the Bronsted-Lowry or Lewis definitions, can accommodate reactions in non-aqueous solutions or even in the gas phase.