When NH3 (ammonia) and HCl (hydrochloric acid) react, they undergo a chemical reaction to form ammonium chloride (NH4Cl). The reaction can be represented as follows:

NH3(g) + HCl(g) → NH4Cl(s)

In this reaction, ammonia acts as a base, accepting a proton (H+) from hydrochloric acid, which acts as an acid. The resulting product, ammonium chloride, is a white, crystalline salt that is soluble in water. The reaction between ammonia and hydrochloric acid is a classic example of an acid-base reaction, in which an acid and a base react to form a salt and water.