Why Can One Substance Have Both a Conjugate Acid and Conjugate Base?

In chemistry, a substance can exhibit both acidic and basic behavior depending on the context. A classic example is the amphiprotic substances, such as water (H₂O) or bicarbonate (HCO₃^–). These substances can either donate or accept protons (H⁺ ions), which is why they can serve as both conjugate acids and conjugate bases.

When a substance acts as an acid, it donates a proton, forming its conjugate base. For instance, when bicarbonate (HCO₃^–) donates a proton, it transforms into carbonate (CO₃^2-), its conjugate base:

HCO₃^– → H⁺ + CO₃^2-

Conversely, when bicarbonate acts as a base, it accepts a proton to form its conjugate acid, carbonic acid (H₂CO₃):

HCO₃^– + H⁺ → H₂CO₃

This dual capacity is a key feature of amphiprotic substances and explains how they can participate in various chemical reactions as either an acid or a base, depending on what other substances they are interacting with in a given environment.