In the reaction: NH4+ + H2O ⮀ NH3 + H3O+ water is acting as a(n)

Water acts as a Brønsted-Lowry Base.

Explanation:

Hello,

In this case, for the theories describing acids, we say that a Brønsted-Lowry Acid is such substance which is able to donate a H⁺ and a Brønsted-Lowry base is such substance which is able to accept a donated H⁺.

In such a way, since hydronium (H₃O⁺) is a product, meaning that water received one H⁺, we conclude water is acting as a Brønsted-Lowry base. Moreover, ammonium ion acts as Brønsted-Lowry Acid.

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