What makes something a lewis acid or a lewis base?
Gilbert Lewis's definition hinges on donation or acceptance of electrons. The Lewis definition says that an acid accepts electrons and a base donates electrons.
The way I remember which is which is by associating Lewis bases with Brønsted-Lowry bases, and Lewis acids with Brønsted-Lowry acids---whenever a proton transfer is involved.
I find Brønsted-Lowry (BL) acids/bases to be easier to remember; BL acids donate protons. Naturally you would expect a general acid
As a result, I tend to associate the BL acid with being a Lewis acid as well, because a BL acid will usually have to accept electrons in order to donate protons.
We can see that in the following reaction:
#"NH"_4^(+) + "OH"^(-)(aq) -> "NH"_3 + "H"_2"O"#
Alternatively, a reaction mechanism depicting it shows:
Hydroxide donated the electrons, which is the opposite behavior to a Lewis acid, so it must be a Lewis base (and since it acquires a proton, it is also a BL base!).
What then results is the formation of ammonium's conjugate base,
(Based on the pKas of ammonium and water, the equilibrium is heavily favored towards ammonia and water, which is a convenient way of utilizing ammonia in lab!)