All the chemical compounds can be categorized as acids, bases, and neutral compounds.
There are different theories, namely Arrhenius Theory, Brønsted-Lowry Theory, Lewis Theory, which define the acids and bases. Each successive theory becoming more general; Lewis Theory is the most generalized one of the three.
Neutral compounds could be salts, hydrocarbons like ethanol, oxides, and many more. A salt is always a result of a reaction between an acid and base.
The difference between a base and an alkali is that an alkali is a base which are soluble in water. In other words, all alkalies are bases but not all bases are alkalies.
I'm sure like bases there would be many acidic compounds which are not soluble in water. Please correct me if I'm wrong.
Is litmus test really a defining criterion for classification of a compound as an acid or base? I believe that to perform litmus test, the compound should be soluble in water, and we have already agreed that not all bases and acids form aqueous solution. It means that litmus test is not a main criterion for the classification. It is something of secondary importance. Do I make sense?
Likewise, general properties given in school chemistry books are also not primary criteria. Stated differently, an acid doesn't always need to taste sour, turn litmus red, and a base doesn't have to feel slippery or turn litmus blue. Is what I'm saying correct?
Boron trifluoride is classified an acid. Boron has three electrons in its outer shell and it forms three covalent bonds with three fluorine atoms which give it total sum of six electrons. It's still deficient of two electrons and therefore it can accept an electron pair so it's Lewis acid.