12.3 Amphoteric Substance
A substance which can act either as an acid or a base, depending on the nature of the other substance is called an amphoteric substance or amphiprotic.
Water can accept a proton and can act as a base. Similarly it can donate a proton and act as an acid.
HCl (g) + H2O H3O+
+ Cl-
 
Base
NH3
(g) + H2O NH4+
+ OH-
Acid
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