Water (H₂O) is considered amphiprotic. This means it can act as both an acid and a base, depending on the situation.

Here's how:

* As an acid: Water can donate a proton (H⁺) to a base, leaving behind a hydroxide ion (OH⁻). For example, when water reacts with ammonia (NH₃), it acts as an acid:

```

H₂O + NH₃ ⇌ NH₄⁺ + OH⁻

```

* As a base: Water can accept a proton (H⁺) from an acid, forming a hydronium ion (H₃O⁺). For example, when water reacts with hydrochloric acid (HCl), it acts as a base:

```

H₂O + HCl ⇌ H₃O⁺ + Cl⁻

```

So, water is not strictly an acid or a base; its behavior depends on the other substance it's interacting with.