Amphoteric nature of water - acid base characteristics

Water is a weak electrolyte and therefore it does not dissociate easily such as strong acid or base. Water molecule ionizes to a small extent to give hydronium ion (H₃O⁺ and OH^-) ion.

Therefore pure water also have small amount of ions. So pure water has a small but measurable electrical conductivity (a current can flow in water due to existing of ions).

Amphoteric behavior of water

Amphoteric behavior means, a compound can act as an acid and base. According to the "Lowry bronsted concept"

Acid - loses a proton (H⁺)
Bases - accepts a proton

Now we know pure water also have H₃O⁺ and OH^- ions.

Amphoteric metals, compounds and behavior

Water and acid reaction

Now we consider, behavior of HCl and H₂S with H₂O. Water molecule accepts a proton from HCl and hydronium ion is formed as the result.

Water and HCl

Hydrochloric acid (HCl) is a strong acid and dissolve very well in water. One electron pair of oxygen atom of water molecule attacks H atom in the HCl molecule because that H atom is positively polarized. So HCl dissociates completely by giving H₃O⁺ ion and Cl^- ion. In this example water molecule behave as a base.

Water and hydrogen sulfide

Water and bases reaction

Water molecule donate a proton to the base to form hydroxyl ion.

Water and ammonia reaction

Ammonia takes a proton from water molecule and forms ammonium ion (NH₄⁺) and hydroxyl ion (OH^-).

Why is water amphoteric

Water is a polar covalent compound. It dissociate slightly in water to H⁺ and OH^- ions. Due to exist of H⁺ ion, it shows acidic characteristics. Also due to exist of OH^- ion, it can take H⁺ ions to show basic characteristics.

What do you understand by amphoteric nature of water

Water can react with both acids and bases like other amphoteric elements and compounds.