So we will take advantage of the fact that we are working with experimental work - there is always an error, and if x is smaller than our error, then we can simplify. It turns out that this depends on the stoichiometry. So far our discussion has dealt only with homogeneous systems, that is all of the components are in the same phase. What about heterogeneous systems where the components occupy different phases. There are three major definitions.
Acids are proton donors.
Bases are proton acceptors. Note that there is no restriction as to solvent, and many substances besides hydroxide ion can contribute basicity.