I know we usually called those OH- bases. So there is another type of base? (base on NH3-)
There are more definitions of acid/base and each one of them covers a different area of substances. When you talk about dissociation with the production of OH
- you think at the Arrhenius definition of base, which is sometimes correct, but fails to explain a wide number of phenomena, such as the basic behaviour of NH
3 in our case That's why there's a better definition (which also includes Arrhenius acids and bases), created by Brönsted and Lowry (if I remember well they worked independently and got the same results more or less at the same time): an acid is a chemical species which can lose a proton (H
+) and a base can gain (or "accept") it. Considering the ammonia behaviour in water we can easily see that it's a base according to this theory (and in fact a NH
3 solution is slightly basic):
NH
3+H
2O ⇋ NH
4++OH
-And as your book says "most of the NH3 molecules remain non-ionized at equilibrium and ammonia is therefore a weak base", which simply means that the equilibrium I've written is shifted towards left