Is Water (H₂O) a Strong or Weak Acid or Base?
Is Water (H₂O) a strong or weak acid or base?
Water is amphiprotic - it can act as both an acid (donating H+ to form OH-) and a base (accepting H+ to form H3O+). This dual nature is central to acid-base chemistry.
| Formula | H₂O |
| Name | Water |
| Category | Amphiprotic |
| pKa | 15.7 |
| Conjugate | Hydroxide ion (OH⁻) |
| Key Concept | Amphiprotic, autoionization |
Definition
Water is amphiprotic - it can donate a proton (acting as an acid) or accept a proton (acting as a base). In pure water, two water molecules undergo autoionization: H2O + H2O <-> H3O+ + OH-.
Acidic Proton / Active Site
Either O-H hydrogen can be the acidic proton. Water donates a proton to a strong base like NaOH or NaNH2, forming OH-.
Conjugate Pair
As an acid: H2O donates H+ to become OH- (hydroxide, the conjugate base). As a base: H2O accepts H+ to become H3O+ (hydronium, the conjugate acid).
Strength Classification
Water has a pKa of 15.7, making it a very weak acid. It only donates a proton to very strong bases. Its Kw = 10^-14 means only 1 in 10 million water molecules is ionized at any time.
See acidic protons, conjugate base overlays, and pKa labels on interactive 3D molecules.
Explore Water's Acid-Base Properties in 3D