In chemistry, a base is a compound that is the opposite of an acid in the sense that it will neutralize an acid. Common bases include compoundss such as some metal oxides and hydroxides, and ammonia.
An acid "donates" H+ ions to the solution, while a base "accepts" H+ ions _or_ donates OH- ions. Both of those actions will decrease the hydrogen ion (H+) concentration, and thus increase pH (-log[H+])
Soluble bases (alkalis) produce hydroxyl ion (OH-) in aqueous solution and have a pH above 7.
Example:
The amino group (NH2) acts as a base by accepting a H+ ions from the solution. It does this by forming a coordiate covalent bond with the unshared pair of electrons belonging to the nitrogen atom. This decreases the hydrogen ion concentration.
Sodium hydroxide (NaOH) decomposes into Na+ and OH-, lowering the hydrogen ion concentration because the hydroxide ion will accept hydrogen ions to form water.
See also: acid-base reaction theories. alkaline foods