According to Bronsted-Lowry concept, an amphoteric substance can both donate and accept a proton (H+). In other words, it can act as both an acid and a base.

Ammonia (NH3) can act as a Bronsted base by accepting a proton from an acid. For example, when ammonia reacts with hydrochloric acid (HCl), it forms ammonium chloride (NH4Cl) and water (H2O):

NH3 + HCl → NH4+ + Cl- + H2O

In this reaction, ammonia accepts a proton from HCl and becomes ammonium ion (NH4+), which is the conjugate acid of ammonia.

Ammonia can also act as a Bronsted acid by donating a proton to a base. For example, when ammonia reacts with sodium hydroxide (NaOH), it forms sodium amide (NaNH2) and water (H2O):

NH3 + NaOH → NaNH2 + H2O

In this reaction, ammonia donates a proton to NaOH and becomes the amide ion (NH2-), which is the conjugate base of ammonia.

Therefore, ammonia is amphoteric according to Bronsted-Lowry concept because it can both donate and accept a proton.