There are several methods of defining acids and bases. While these definitions don’t contradict each other, they do vary in how inclusive they are. Antoine Lavoisier, Humphry Davy, and Justus Liebig also made observations regarding acids and bases, but didn’t formalize definitions.

Svante Arrhenius

* acids produce H+ ions in aqueous solutions
* bases produce OH- ions in aqueous solutions
* water required, so only allows for aqueous solutions
* only protic acids are allowed; required to produce hydrogen ions
* only hydroxide bases are allowed

Johannes Nicolaus Brønsted – Thomas Martin Lowry

* acids are proton donors
* bases are proton acceptors

* aqueous solutions are permissible
* bases besides hydroxides are permissible
* only protic acids are allowed

Gilbert Newton Lewis

* acids are electron pair acceptors
* bases are electron pair donors
* least restrictive of acid-base definitions

Properties of Acids

* taste sour (don’t taste them!)… the word ‘acid’ comes from the Latin acere, which means ‘sour’
* acids change litmus (a blue vegetable dye) from blue to red
* their aqueous (water) solutions conduct electric current (are electrolytes)
* react with bases to form salts and water
* evolve hydrogen gas (H2) upon reaction with an active metal (such as alkali metals, alkaline earth metals, zinc, aluminum)

Properties of Bases

* taste bitter (don’t taste them!)
* feel slippery or soapy (don’t arbitrarily touch them!)
* bases don’t change the color of litmus; they can turn red (acidified) litmus back to blue
* their aqueous (water) solutions conduct and electric current (are electrolytes)
* react with acids to form salts and water