Question

Anhydrous hydrogen chloride is not an acid but its aqueous solution is a strong acid. Explain.

Answer 1

Anhydrous hydrogen chloride is in gaseous state at room temperature; therefore, it cannot dissociate to form H₃O⁺ and Cl⁻ ions. But when this gaseous hydrogen chloride is dissolved in water, it becomes hydrochloric acid. On dissolution in water, it dissociates completely to form hydronium (${\mathrm{H}}_3{\mathrm{O}}^{+}$) and chloride (${\mathrm{Cl}}^{-}$) ions. Thus, it is considered a strong acid.

$$\begin{gathered} \mathrm{HCl}+{\mathrm{H}}_2\mathrm{O}\leftrightarrow {\mathrm{H}}_3{\mathrm{O}}^{+}+{\mathrm{Cl}}^{-} \\ \mathrm{Hydrochloric}\mathrm{Water}\mathrm{Hydronium}\mathrm{ion}\mathrm{Chloride}\mathrm{ion} \\ \mathrm{acid} \end{gathered}$$