Arrhenius Theory of Acids and Bases

In 1887, the Swedish scientist Svante Arrhenius proposed a theory to explain the behaviour of acids and bases in water. This theory provided a simple and clear definition of acids and bases based on their behaviour in aqueous solutions.

1. Acids: An acid is a substance that, when dissolved in water, increases the concentration of H⁺ ions (protons) in the solution. These H⁺ ions are often represented as forming H₃O⁺ ions (hydronium ions) because they combine with water molecules.

\[\mathrm{HCl~(g)}\xrightarrow[(\mathrm{dissociation})]{\mathrm{water}}\quad\mathrm{H^{+}(aq)}+\mathrm{Cl^{-}(aq)}\]

\[\mathrm{H}_2\mathrm{SO}_4(l)\xrightarrow[(\mathrm{dissociation})]{\mathrm{water}}\mathrm{H}^+(\mathrm{aq})+\mathrm{HSO}_4^-(\mathrm{aq})\]

\[\mathrm{HSO}_4^-(\mathrm{aq})\xrightarrow{(\text{dissociation})}\mathrm{H}^+(\mathrm{aq})+\mathrm{SO}_4^{2-}(\mathrm{aq})\]

2. Bases: A base is a substance that, when dissolved in water, increases the concentration of OH⁻ ions (hydroxide ions) in the solution.

\[\mathrm{NaOH~(s)~}\xrightarrow[(\mathrm{dissociation})]{\mathrm{water}}\mathrm{Na^+(aq)}+\mathrm{OH^-(aq)}\]

\[\mathrm{Ca(OH)}_{2}(\mathrm{s})\quad\frac{\mathrm{water}}{(\text{dissociation})}\quad\mathrm{Ca}^{2+}(\mathrm{aq})+2\mathrm{OH}^{-}(\mathrm{aq})\]