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.
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+).