Acid Base Defined by Behavior

• Some substances—such as water—can act as acids or bases either accepting or donating a proton.
• Substances that can act as acids or bases are amphoteric.
• What happens when an equation representing Brønsted–Lowry acid–base behavior is reversed?
• Now NH4+ is the proton donor (acid) and OH− is the proton acceptor (base).
• What was the base (NH3) has become the acid (NH4+), and what was the acid (H2O) has become the base (OH−).
• NH3 and NH4+ are often referred to as a conjugate acid–base pair, two substances related to each other by the transfer of a proton.