Here's why it's possible:

* Acids donate protons (H+)

* Bases accept protons (H+)

A molecule can act as both an acid and a base depending on the reaction environment. This means it can either donate or accept a proton, depending on what it's reacting with.

Examples of Amphiprotic Species:

* Water (H2O):

* Can act as an acid: H2O → H+ + OH-

* Can act as a base: H2O + H+ → H3O+

* Bicarbonate ion (HCO3-):

* Can act as an acid: HCO3- → H+ + CO32-

* Can act as a base: HCO3- + H+ → H2CO3

* Amino acids: These molecules contain both acidic (carboxyl) and basic (amino) groups, making them excellent examples of amphiprotic behavior.

Key Takeaway:

The behavior of a chemical as an acid or base is not an inherent property, but rather a function of its reaction environment and the species it interacts with. Amphiprotic molecules demonstrate this versatility, showcasing their ability to act as both proton donors and acceptors.