Water is amphoteric meaning that it can act as both an acid and a base. The water molecule has hydrogen atoms and, for that reason, could act as an acid in a reaction. The amphoteric characteristic of water is important in the acid-base properties of aqueous solutions.
HNO3 (aq) + H2O (I) ↔ NO3– (aq) + H3O+ (aq)
In the forward reaction, HNO3 donates a proton to H2O. Thus, HNO3 is acid and H2O is here a base.
In the backward reaction, NO3 accepts a proton from H3O+. Thus, NO3 is acid and H2O is a base.
NH3 (aq) + H2O (I) → NH4+ (aq) + OH– (aq)
In this forward reaction, NH3 accepts a proton from H2O. Thus, NH3 is a base and H2O is an acid. In the backward reaction, NH4+ donates a proton to the OH–. Here, the OH– is a base and NH4+ is an acid. The oxygen atom in the water molecule has two lone pairs, one of which could be used to form a bond with an (H+), and, therefore, the water molecule could act as a base in a reaction. Since water has the potential to act both as an acid and as a base, water is amphoteric.
Amphoteric behavior means the substance can react with both acid and base. According to the “Lowry bronsted concept” – bases stronger than water, lean-to allow proton from it. Thus by donating a proton, water acts as an acid.
H2O + NH3 (a base) → NH4+ + OH– [Bases – accepts a proton]
H2O + HCl (an acid) → H+ + Cl– [Acid – loses a proton (H+)]
Because water can react to both acid and base, we say it is amphoteric. Thus water can donate as well as accept proton and show amphoteric nature.
One of the two water molecules acts as a Bronsted-Lowry acid and donates a proton to the other water molecule, which consequently acts as a Bronsted-Lowry base.
Water can act as an acid or as a base in various chemical reactions, the most common two examples involving the ammonia and hydrochloric acid reactions.
NH3 (aq)+H2O (l) ⇌ NH4+ (aq) + OH– (aq)
Here water acts as a Bronsted-Lowry acid because it donates a proton to ammonia.
HCl (g) + H2O (l) ⇌ H3O+ (aq) + Cl– (aq)
This time water acts as a Bronsted-Lowry base because it accepts a proton from hydrochloric acid.
To be exact, in reactions that involve the transfer of protons, the term is actually amphiprotic.