Acids produce hydrogen ions, H+, in water. Bases produce hydroxide ions, OH-.
A hydrogen ion is a bare proton that associates with a water molecule so the H+ ions produced by an acid exist as H3O+ ions:
`H^+ + H_2O -> H_3O^+`
There are three accepted theories defining acids and bases:
1. The Arrhenius theory defines an acid as substance that dissociates to produce H+ ions in solution and a base as a substance that dissociates to produce OH- ions in solution, as per the answer to your question.
2. The Bronsted-Lowery theory defines an acid as a proton donor and a base as a proton acceptor. (Remember that a proton is the same as an H+ ion.)
3. The Lewis theory defines an acid as an electron pair acceptor and a base as an electron pair donor.
Here are a few examples of these definitions:
HCl is both an Arrhenius acid and a Bronsted-Lowry acid. It dissociates to produce hydrogen ions in aqueous solution, and it donates a proton to a base. These are two different ways of describing the same behavior.
Bases that contain the hydroxide ion OH- are both Arrhenius and Brønsted-Lowry bases because they dissociate to produce OH- ions that "accept" or bond to protons:
`H^+ + OH^(-) -> H_2O`
H+ is a Lewis acid because it will "accept" or share a non-bonding pair of electrons on another atom or ion. For example, it accepts an electron pair from the Lewis base F-:
`H^+ + F^(-) -> HF`
Ammonia, NH3, is an example of all three types:
`NH_3 + H_2O --gt NH_4^+ + OH^-`
Ammonia produces OH- by reacting with water. It can also be said that it accepts a proton from water and that it donates an electron pair to an H+ ion.