Final answer:
Substances like water that can act as a Bronsted acid in some reactions and as a Bronsted base in others are called amphoteric substances or amphiprotic compounds.
Step-by-step explanation:
Substances such as water that can act as either a Bronsted acid or a Bronsted base, depending on the reaction, are called amphoteric substances. Water is a common example of an amphoteric substance; it can accept a proton and act as a base, as in its reaction with HCl, or donate a proton to act as an acid, as in the reaction with NH3 (ammonia). The ability to behave as either an acid or a base in a chemical reaction is a characteristic of amphiprotic compounds.
The Bronsted-Lowry theory defines acids and bases in terms of their ability to donate or accept protons (H+ ions). For example, when water acts as a base in the presence of a stronger acid like HCl, it accepts a proton to form hydronium ions. Conversely, when water acts as an acid in the presence of a stronger base like ammonia, it donates a proton and forms hydroxide ions. This dual capability underpins the concept of amphoteric substances in the broader context of acid-base chemistry.