Answer:
I’ll give you two possible conditions:
(1) An acid is present in an aqueous solution. The acid will donate a proton to the water to form hydronium. (That’s not really what happens, but that’s how we usually think of it.)
(2) The autoionization of water: in a pure water solution (or not a pure solution, doesn’t matter), one water molecule donates a proton to another water molecule, forming equal numbers of hydronium and hydroxide ions.
