Figure 2. Water molecule behaving as an acid. When water behaves as a base, it accepts a proton and becomes a hydronium ion. Figure 3. Water molecule behaving as a base. When an acid or base is added ...

the mutual neutralization of hydronium (H 3 O⁺) and hydroxide (OH⁻) ions. This reaction, essential to acid-base chemistry, is typically understood to yield two water molecules (H 2 O).