# What must an Arrhenius acid contain?

Aug 6, 2018

An Arrhenius acid is a substance whose aqueous solutions give rise to hydrogen ions or protons.

#### Explanation:

And thus we may identify as an acid a species that undergoes the following reaction with water...

$H X \stackrel{{H}_{2} O}{\rightarrow} {H}^{+} + {X}^{-}$

The acidium ion, ${H}^{+}$, is an hypothetical species, and is more commonly represented as hydronium ion, i.e. ${H}_{3} {O}^{+}$, i.e.

$H X \left(a q\right) + {H}_{2} O \left(l\right) \rightarrow {H}_{3} {O}^{+} + {X}^{-}$

Both ${H}^{+}$, and ${H}_{3} {O}^{+}$ are labels of convenience (the latter is more common these days. The actual acidium ion is PROBABLY a cluster of 3-4 water molecules with an EXTRA proton, ${H}^{+}$...to give ${H}_{7} {O}_{3}^{+}$ or ${H}_{9} {O}_{4}^{+}$. The proton is presumed to tunnel from cluster to cluster, we write ${H}_{3} {O}^{+}$ without loss of generality.

And so an Arrhenius acid must contain an hydrogen ion that can be transferred to solvent....