Question

What is an acid?

Answer 1

See below.

Explanation:

There are three definitions of acids that could be found in literature that have been given by four scientists (2 scientists gave the same definition, Brønsted and Lowry):

Arrhenius definition:
Acids: produce hydrogen ions (`H^+`) in aqueous solution. Arrhenius acids include compounds such as `HCl`, `HCN`, and `H_2SO_4` that ionize in water to give the `H^+` ion.

Brønsted-Lowry definition:
Acids: compounds that are proton (`H^+`) donors.
This is similar to the Arrhenius definition but it is more general and it is not limited to aqueous solution only.

Lewis definition:
Acids: compounds that are an electron pair acceptors.

Acids & Bases | Nature, Strength & pH Scale.