Here's why:

* Acids donate protons (H+) in solution.

* Bases accept protons (H+) in solution.

Pure water undergoes a process called auto-ionization, where a small fraction of water molecules spontaneously ionize into hydronium ions (H3O+) and hydroxide ions (OH-):

```

2H2O(l) ⇌ H3O+(aq) + OH-(aq)

```

The concentration of both H3O+ and OH- ions in pure water is equal at 10^-7 M at 25°C. This is why pure water has a pH of 7, which is the neutral point on the pH scale.