Are all liquids either an acid or base?

Answer 1

In the absolute sense of the words "acid" and "base," no. Pure water is neither. However, you can also have "relative acids" and "relative bases," which are solutions capable of donating protons and accepting protons, respectively.

Answer 2

Any aqueous, or perhaps alcohol based liquids will have a pH associated with it, which could be neutral, acidic, or alkaline. Water itself isn't a particularly good buffer, so very slight perturbations in the pH are quite noticeable.

However, acids and bases can be thought of as the ability to donate or accept protons (Hydrogen ions).

So, most non-polar hydrocarbons would not fall under either acids or bases, and wouldn't be particularly good acids, bases, or solvents for (most) acids and bases. And hydrogens bonded to carbon are essentially non acidic. Likewise the liquids of many metals might not really refer to acids or bases. For example liquid Mercury... or for that matter, molten iron.

Now, there are some exceptions... for example the acetylene molecule (C2H2 or HCCH) could be considered a very weak acid, with its conjugate base in ionic form HCC- being one of the strongest bases.