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In chemistry, hydronium is the common name for the aqueous cation H
, the type of oxonium ion produced by protonation of water. It is the positive ion present when an Arrhenius acid is dissolved in water, as Arrhenius acid molecules in solution give up a proton (a positive hydrogen ion, H+) to the surrounding water molecules (H2O). Hydronium is the cation that forms from water in the presence of hydrogen ions. These hydrons do not exist in a free state: they are extremely reactive and are solvated by water. Hydronium is very acidic: at 25 °C, its pKa is −1.7. It is the most acidic species that can exist in water (assuming sufficient water for dissolution): any stronger acid will ionize and protonate a water molecule to form hydronium.
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Hydronium is what you get when you put water and hydrogen ions together, forming H3O+. Hydronium is the simplest form of oxonium, which is any ion that contains the trivalent oxygen cation. Hydronium is also known as hydroxonium. As with many species in chemistry, the nomenclature isn't the same everywhere.

 Hydronium is found in interstellar clouds and in the tails of comets. Interstellar hydronium probably forms as a result of chemical reactions following the ionization of H2 into H2+. Research is ongoing to elucidate the nature of the reactions.