Advanced Placement (AP) Chemistry Practice Exam

What is a common characteristic of amphoteric substances?

1. They can only donate protons.

2. They can only accept protons.

3. They can both gain and lose protons.

4. They are always strong acids.

The correct answer is: They can both gain and lose protons.

Amphoteric substances are unique in that they possess the ability to act as both acids and bases. This means they can both gain protons (acting as a base) and lose protons (acting as an acid), which is exactly what makes them amphoteric. Examples of such substances include water and bicarbonate ions, which can either donate a proton to become a negatively charged species or accept a proton to form a positively charged species. This dual behavior is essential in various chemical reactions, including acid-base equilibria and buffer systems. The other options do not accurately describe amphoteric behavior: some substances can only donate protons, while others can only accept them, and amphoteric substances are not defined by strength; there are both strong and weak amphoteric substances. This versatility is a key characteristic of amphoteric substances, highlighting their importance in chemistry.