Acid-base theories: Arrhenius, Brönsted-Lowry and Lewis

Arrhenius theory.
* Acid. It is any substance that in aqueous solution produces hydrogen ions (protons).

* Base. Substance that in aqueous solution produces OH (hydroxide) ions. And if you combine an acid with a base, it produces salt and water.
Lewis theory.

* Acid. A substance that accepts a pair of electrons and is called an electrophile.

* Base. A substance that gives up a pair of electrons and is called a nucleophile.
Brönsted-Lowry theory.
They established that in a redox reaction there is proton transfer (proton exchange theory).
* Acid. It is an ion that gives up a proton.

* Base. It is an ion that accepts a proton.
Classification by conductivity: strong and weak.
* Strong acid: Substance that in aqueous solution easily loses its proton.
* Weak acid: Substance that in aqueous solution loses its proton with difficulty, does not dissociate easily.
* Strong base: That which in aqueous solution dissociates easily.

* Weak base: That which in aqueous solution does not dissociate easily.