I'm not a science student - quite a layman. So, please keep your replies simple and straightforward. Thanks. Why are there more than one theory on acid-base? Why isn't one theory enough? Are these theories inter-compatible? Does one theory try to counter the limitations of earlier theory(ies)? It is normally told that an acid tastes sour (something like lemon juice) and turns litmus red. Is this always true, no matter what? Please guide me.