Water (H2O) is unique in that it is both an acid and a base. This property is known as being amphiprotic. According to the provided reference, water can both accept and donate hydrogen ions (H+), enabling it to act as either an acid or a base.
How Water Acts as an Acid and Base
| Characteristic | Acidic Role | Basic Role |
|---|---|---|
| Action | Donates a hydrogen ion (H+) | Accepts a hydrogen ion (H+) |
| Result | Forms a hydroxide ion (OH-) | Forms a hydronium ion (H3O+) |
| Description | Acts as a proton donor | Acts as a proton acceptor |
| Example Reaction | H2O + NH3 ⇌ OH- + NH4+ (water donates H+) | H2O + HCl ⇌ H3O+ + Cl- (water accepts H+) |
Understanding Amphiprotic Nature
- Self-ionization: Water can undergo self-ionization, a process where one water molecule donates a proton to another, forming hydronium (H3O+) and hydroxide (OH-) ions. This shows its ability to act both as an acid and a base in the same reaction.
- Dynamic Equilibrium: In pure water, the concentrations of H3O+ and OH- are equal, making it neutral. This dynamic equilibrium constantly shifts as other substances are added.
Examples and Practical Insights
- Acidic Solutions: When acids are added to water, they donate H+ ions, increasing the concentration of H3O+ ions and making the solution acidic. In these scenarios, water acts as a base by accepting these ions.
- Basic Solutions: When bases are added to water, they accept H+ ions, increasing the concentration of OH- ions and making the solution basic. In these cases, water acts as an acid by donating H+ ions.
- pH Scale: The pH scale measures the acidity or basicity of a solution. Pure water has a pH of 7, indicating a neutral solution with equal concentrations of H3O+ and OH- ions.
Therefore, because H2O can act as both a proton donor and a proton acceptor, it's considered both an acid and a base. Its amphiprotic nature is fundamental in many chemical reactions and biological processes.
