What defines an amphoteric substance?

An amphoteric substance is defined by its ability to act as both an acid and a base depending on the circumstances of the chemical reaction it is involved in. This means that it can either donate protons (acting as an acid) or accept protons (acting as a base).

For instance, substances like water and amino acids are classic examples of amphoteric compounds. In a reaction with a strong base, water can donate a proton, while in the presence of a strong acid, it can accept a proton.

This definition is essential in understanding acid-base chemistry because amphoteric substances can help facilitate various reactions, functioning according to the needs of the system. The other options do not capture this dual capacity, focusing instead on singular roles in acid-base chemistry.