What characteristic defines an amphoteric substance?

An amphoteric substance is characterized by its ability to act as both an acid and a base, depending on the environment or the substances it is reacting with. This dual capability arises from the presence of functional groups or ions in the substance that can either donate protons (H⁺ ions) when acting as an acid or accept protons when functioning as a base.

For example, water is a classic amphoteric substance. It can donate a hydrogen ion to become a hydroxide ion (acting as an acid) or accept a hydrogen ion to become a hydronium ion (acting as a base). This property is significant in various chemical reactions and is fundamental to understanding acid-base chemistry.

Understanding this concept is crucial for laboratory analysts working with water samples, as it helps in determining the behavior of different substances in varying pH conditions.