(N/A) Definition: According to the Arrhenius theory,acids are substances that dissociate in water to give hydrogen ions $(H_{(aq)}^{+})$. According to the Arrhenius theory,bases are substances that produce hydroxyl ions $(OH_{(aq)}^{-})$.
General formula for an acid is $HX$ and for a base is $MOH$.
Ionization of acid $HX$ in aqueous solution:
$HX_{(aq)} \longrightarrow H_{(aq)}^{+} + X_{(aq)}^{-}$
$OR$
$HX_{(aq)} + H_{2}O_{(l)} \longrightarrow H_{3}O_{(aq)}^{+} + X_{(aq)}^{-}$
Note: $A$ bare proton $H^{+}$ is very reactive and cannot exist freely in aqueous solutions. Thus,it bonds to the oxygen atom of a water molecule to form a hydronium ion $(H_{3}O^{+})$.
$H_{(aq)}^{+} + H_{2}O_{(l)} \longrightarrow H_{3}O_{(aq)}^{+}$
Ionization of base $MOH$ in aqueous solution:
$MOH_{(aq)} \longrightarrow M_{(aq)}^{+} + OH_{(aq)}^{-}$
Limitations of the Arrhenius concept:
$(i)$ It is applicable only to aqueous solutions.
$(ii)$ It does not account for the basicity of substances like ammonia $(NH_{3})$ which do not possess a hydroxyl group.
$(iii)$ It does not explain the stability of the $H^{+}$ ion.