In chemistry, a base is a substance that, in aqueous solution, is slippery to the touch, tastes bitter, changes the colour of indicators (e.g., turns red litmus paper blue), reacts with acids to form salts, and promotes certain chemical reactions (base catalysis).
Examples of bases are :
- the hydroxides of the alkali and
- alkaline earth metals (NaOH, Ca(OH)2, etc.).
Such substances produce hydroxide ions (OH-) in aqueous solutions, and are thus classified as Arrhenius bases.
For a substance to be classified as an Arrhenius base, it must produce hydroxide ions in solution—in order to do so, Arrhenius believed the base must contain hydroxide in the formula.
This makes the Arrhenius model limited, as it cannot explain the basic properties of aqueous solutions of ammonia (NH3) or its organic derivatives (amines).
In the more general Brønsted–Lowry acid–base theory, a base is a substance that can accept hydrogen ions (H+)—otherwise known as protons.
In the Lewis model, a base is an electron pair donor.