Why do acids not show acidic behaviour in the absence of water?
Acids do not show acidic behaviour in the absence of water because the separation of hydrogen ions (H⁺) from acid molecules cannot occur without water. For instance, in HCl, the H⁺ ion separates from the HCl molecule only when water is present.
Hydrogen ions cannot exist alone; they must combine with water molecules to form hydronium ions (H₃O⁺). The reaction occurs as: HCl + H₂O → H₃O⁺ + Cl⁻. Since the acidic properties of acids are due to the presence of H⁺(aq) or hydronium ions in solution, acids fail to exhibit acidic behaviour in the absence of water.
Explanation
The textbook context from Activity 2.9 demonstrates that dry HCl gas does not change the colour of dry litmus paper, while HCl solution does. This proves that acids require water to dissociate and produce hydrogen ions. The key concept is that H⁺ ions cannot exist independently—they must combine with water to form hydronium ions (H₃O⁺). The acidic character is solely due to these ions in aqueous solution.