I am aware of the definitions of these words. but, honestly, we can expect few questions on these two tags. Hence can we have one as the synonym of the other, and retain acid-base tag?
Yep. Tags are for filtering, so we ought to synonymify acid,base,acidic,basic,basicity/etc to acid-base. (we only really need to make two-ish synonyms, the rest will be covered since the autocomplete will pop up the synonyms)
I can think of enough questions on this though, so we ought to have the tag.
Though, then again, there's not much use for filtering acid-base questions. I still say we keep the tag with synonyms-- acidic/basic nature is a pretty extensive topic (not that I'd know much about it)