This means that they can either donate or accept protons, depending on the environment they are in.

In an acidic environment, they will act as a base and accept protons.

In a basic environment, they will act as an acid and donate protons.

For example, water is an amphoteric substance. In pure water, there are an equal number of hydrogen ions (H+) and hydroxide ions (OH-). This is because water can undergo a process of self-ionization, where a water molecule splits into a hydrogen ion and a hydroxide ion. When water is mixed with an acid, the hydrogen ions from the acid will react with the hydroxide ions from the water to form more water molecules. This will decrease the number of hydroxide ions in the water and make the solution more acidic. When water is mixed with a base, the hydroxide ions from the base will react with the hydrogen ions from the water to form more water molecules. This will decrease the number of hydrogen ions in the water and make the solution more basic.