Question

Why is ammonia considered a base?

Answer 1

It's a Bronsted base because it can accept a proton.

Explanation:

The Brondsted-Lowry definition defines bases as substances that accept protons, or `H^+` ions, in an aqueous solution.

When ammonia, `NH_3`, is placed into an aqueous solution, this reaction will occur:

`NH_3(aq) + H_2O(l) rightleftharpoons NH_4^+(aq) + OH^(-)(aq)`

We can see that `NH_3` is accepting an `H^+` ion from `H_2O`.

The forward reaction doesn't occur that readily, so, although ammonia is a base, it's a weak base.

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If we were to be really specific, ammonia sometimes also behaves as an acid by donating an `H^+` ion.
This would make ammonia amphoteric, which means that it can behave as both an acid and a base.

`NH_3(aq) + H_2O(l) rightleftharpoons NH_2^(-)(aq) + H_3O^(+)(aq)`

BUT, the above reaction (when ammonia is behaving as an acid) happens at a way lower rate than when ammonia accepts a proton, so we call it a base.