Which substance is acting as the brønsted-lowry acid in the following chemical reaction? nh4 + oh- yields nh3 + h2o

According to Bronsted-Lowey concept of Acid and Base, an Acid is any specie which donates proton (H+) while, Base is a substance which accepts proton from acid. In the reaction above, Ammonium ion is acting as Acid and Hydroxyl ion as Base because Ammonium ion donates proton and converts into ammonia a conjugate base, while, hydroxyl ion accepts proton converting into water (a conjugate acid in this case).

The mass has escaped into a gas which you can not see so the law stays true.

Nonmetal are moldable and easily shaped