Chemists often write H+(aq) and refer to the hydrogen ion when describing acid–base reactions but the free hydrogen nucleus, a proton, does not exist alone in water, it exists as the hydronium ion (H3O+) or other forms (H5O2+, H9O4+).
kˈɛmɪsts ˈɔfən ɹˈaɪt ˈeɪtʃ plˈʌs ˈæk ænd ɹᵻfˈɜː tə ðə hˈaɪdɹədʒən ˈaɪɑːn wɛn dᵻskɹˈaɪbɪŋ ˈæsɪd bˈeɪs ɹɪˈækʃənz bˌʌt ðə fɹˈiː hˈaɪdɹədʒən nˈuːklɪəs, ɐ pɹˈoʊtɑːn, dʌznˌɑːt ɛɡzˈɪst ɐlˈoʊn ɪn wˈɔːɾɚ, ɪɾ ɛɡzˈɪsts æz ðə haɪdɹˈoʊniəm ˈaɪɑːn ˈeɪtʃ θɹˈiː ˈoʊ plˈʌs ɔːɹ ˈʌðɚ fˈɔːɹmz ˈeɪtʃ fˈaɪv ˈoʊ tˈuː plˈʌs, ˈeɪtʃ nˈaɪn ˈoʊ fˈoːɹ plˈʌs.