Hydronium is a positively-charged ion that forms from water in the presence of hydrogen ions. The formula for hydronium is H3O+.

Hydronium is present whenever an acid is dissolved in water; the hydrogen ions released by the acid do not exist in a free state. Water will also spontaneously dissociate into hydronium and hydroxide ions, in the following reaction:

The equilibrium of this reaction is far to the left under most conditions, and so pure water will have little hydronium in it. About 1 molecule in 10,000,000 is dissociated.

Since the reaction releases an OH - ion, this ion's alkalinity balances hydronium's acidity, and water remains neutral (a hydronium ion concentration of 1 x 10-7.0 at 298 K).

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