Question

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

Answer 1

In aqueous solution, we REPRESENT the acidium ion as `H^+` or `H_3O^+`.

Explanation:

The acidium ion is the characteristic cation of the solvent; and for water we represent this species as `H^+` or `H_3O^+`. 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_7O_3^+` or `H_9O_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_3O^+`, the `"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 `NH_4^+`, and the characteristic anion is amide ion, `NH_2^-`. In a more acidic medium, say hydrogen fluoride, these become `H_2F^+` and `F^-`.