# What is the acid principle in water? Is there such a thing as H^+?

Nov 14, 2016

In aqueous solution, we REPRESENT the acidium ion as ${H}^{+}$ or ${H}_{3} {O}^{+}$.

#### Explanation:

The acidium ion is the characteristic cation of the solvent; and for water we represent this species as ${H}^{+}$ or ${H}_{3} {O}^{+}$. So what is it really?

As far we know, the acidium species is a cluster of 3 or 4 water molecules, with an EXTRA proton attached to give ${H}_{7} {O}_{3}^{+}$ or ${H}_{9} {O}_{4}^{+}$. The extra proton can be readily exchanged between water clusters, so that it could rapidly tunnel across the solution. If you have ever played rugby, think of a maul where the forwards exchange the pill to the back of the maul and keep driving.

Nowadays, it is common to represent the acidium ion in water as ${H}_{3} {O}^{+}$, the $\text{hydronium ion}$. It is more of a representation than an actual species.

Other solvent systems exist. In liquid ammonia, the characteristic cation, the acid, is $N {H}_{4}^{+}$, and the characteristic anion is amide ion, $N {H}_{2}^{-}$. In a more acidic medium, say hydrogen fluoride, these become ${H}_{2} {F}^{+}$ and ${F}^{-}$.