Acid

There are several definitions of an acid (Arrhenius, Brönsted etc.) but the Lewis definition is the widest. This states that an acid is an electron pair acceptor (and hence possesses an empty orbital).

Conjugate acid and base

For every acid there is a conjugate base and vice versa. A base is therefore an electron pair donor. The strengths of an acid and its conjugate base are related. If an acid is strong, its conjugate base is weak, and reciprocally.

General facts about acids

An acid will colour litmus paper red and have a pH lower than 7.