Answer:
base
Explanation:
In the reaction,
NH₄⁺ + H₂O ----> NH₃ + H₃O⁺
water is acting as an base.
This appears to be an acid-base reaction. Acids donate hydrogen ions. On the reactants side, ammonium (NH₄) is an acid because it has excess hydrogens it wishes to get rid of once it is introduced to water. During the reaction, NH₄ donates a hydrogen to the base (water) and becomes ammonia (NH₃), its conjugate base. As a result, water receives a hydrogen and becomes hydronium (H₃O), its conjugate acid.
NH₄⁺ + H₂O ----> NH₃ + H₃O⁺
acid base conjugate conjugate
base acid
