Water (H2O) is not always an acid, but it can act as one under certain conditions.
Understanding Water's Dual Nature
According to the Bronsted-Lowry definition, an acid is a substance that can donate a proton (a hydrogen ion, H+), and a base is a substance that can accept a proton. Water is considered amphiprotic, meaning it can function as both an acid and a base depending on the reaction.
Water as an Acid
- In the presence of a stronger base, water can donate a proton (H+) and behave like an acid.
- For instance, water reacts with ammonia (NH3), where water acts as the acid by donating a proton to form a hydroxide ion (OH-) and an ammonium ion (NH4+):
H2O + NH3 ⇌ OH- + NH4+
- For instance, water reacts with ammonia (NH3), where water acts as the acid by donating a proton to form a hydroxide ion (OH-) and an ammonium ion (NH4+):
Water as a Base
- In the presence of a stronger acid, water can accept a proton (H+) and behave as a base.
- For instance, water reacts with hydrochloric acid (HCl), where water acts as a base by accepting a proton to form a hydronium ion (H3O+) and a chloride ion (Cl-):
H2O + HCl ⇌ H3O+ + Cl-
- For instance, water reacts with hydrochloric acid (HCl), where water acts as a base by accepting a proton to form a hydronium ion (H3O+) and a chloride ion (Cl-):
Key Takeaways
| Feature | Acid Behavior | Base Behavior |
|---|---|---|
| Proton Role | Donates a proton (H+) | Accepts a proton (H+) |
| Reaction | Reacts with a stronger base | Reacts with a stronger acid |
| Example | Reacts with ammonia (NH3) to form OH- | Reacts with hydrochloric acid (HCl) to form H3O+ |
| Term | Bronsted-Lowry acid | Bronsted-Lowry base |
| Nature | Amphiprotic | Amphiprotic |
Summary
Water is amphiprotic, meaning it can behave as either an acid or a base. Water's acidity depends on its reaction partner. It is not always an acid.
