But if a free hydrogen ion encounters a water molecule, it attacks the unshared electron pairs on the oxygen in the water molecule and forms a hydronium ion, H3O+. The chemical bond that forms between the water and hydrogen ion is covalent and very strong. In an aqueous solution, essentially all of the H+ exists as H3O+.
When people write "H+(aq)" in a chemical equation, it's understood that they really mean "H3O+(aq)". Writing hydrogen ions rather than hydronium ions makes neutralization equations a little easier to read and balance.
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Last Revised 08/17/15.URL: http://antoine.frostburg.edu/chem/senese/101/acidbase/faq/print-isolated-h-ions.shtml