Amphoterism

A chemical compound is amphoteric if it can act as both an acid and a base. Amphoteric compounds are essential to life; not only water, but also bicarbonate as part of buffer solutions used to control acidity in the body.

 Example 
Water is amphoteric. A strong acid (such as hydrochloric acid) will react with water to produce a hydronium ion and the acid's conjugate base (such as the chloride ion). In this reaction, water is a base:

Reactions with water as an acid are more complicated. Many compounds known as "strong base" do not react with water at all: instead, they are salts of hydroxide that dissolve in water. Hydroxide is the conjugate base of water, so the two will not react; a stronger base is needed to cause an acid-base reaction. With the right base (such as the methoxide ion), a reaction will occur that makes hydroxide and the conjugate acid (such as methanol).

 Related pages 
 Amphoteric oxide, oxygen compounds that show amphoterism

 References 

Category:Amphoteric compounds