Water molecules transfer protons to form H3O+ and OH-, with Kw of 1e-14 at 25 C.
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Water undergoes a small self-ionization in which one molecule transfers a proton to another: 2H2O <-> H3O+ + OH-. The equilibrium constant Kw equals [H3O+][OH-], which is 1.0 x 10^-14 at 25 C, so in pure water both ion concentrations are 1.0 x 10^-7 M. Because the process is endothermic, Kw rises with temperature, which lowers the pH of neutral water, but the solution remains neutral because [H+] still equals [OH-]. This constant links acidity and basicity and allows pH and pOH to be related. Ignoring it leads to wrong answers when very dilute acids or bases are involved.
Key ideas: 2H2O gives H3O+ + OH-, with Kw = [H+][OH-] = 1.0e-14 at 25 C. In pure water each ion is 1.0e-7 M. The process is endothermic, so Kw rises with temperature: about 5.5e-14 at 50 C and 1.0e-13 near 60 C. Water concentration is constant (about 55.5 M) and is absorbed into Kw.
Common mistakes and connections: Mistake: saying water is neutral only at pH 7. Neutral means [H+] = [OH-], so at 50 C neutral pH is about 6.63. Also, Kw applies to every aqueous solution, not just pure water. Links to pH, pOH and the temperature dependence of equilibrium constants.
Kw rises with temperature, so neutral water has a lower pH when hot (6.13 at 100 C) yet is still neutral because H+ equals OH-. The interactive lesson shows this as a live animation or chart.
At 60 C, Kw is about 9.6 x 10^-14, so neutral water has a pH near 6.5.
Water molecules transfer protons to form H3O+ and OH-, with Kw of 1e-14 at 25 C.
Autoionization of Water is a intermediate topic in Acids, Bases & Electrochemistry. It builds on the vocabulary of beginner topics in this area.
Study these earlier topics in Acids, Bases & Electrochemistry first: Lewis Acids and Bases, Conjugate Acid-Base Pairs, Amphoteric Substances.
The next topics in Acids, Bases & Electrochemistry are pH Scale and pOH and pKw.
Yes. The interactive lesson includes a self-check question on Autoionization of Water, and the Acids, Bases & Electrochemistry quiz draws on every topic in this area.
Autoionization of Water is topic 6 of 105 in Acids, Bases & Electrochemistry.