This guy named Arrhenius concocted the first successful concept of acids and bases. He did this by defining acids and bases according to the effect these substances have on water. The Arrhenius concept of acids and bases is as follows: an acid is a substance that when dissolved in water increases the concentration of the hydrogen ion, H+. A base is a substance that when dissolved in water increases the concentration of the hydroxide ion, OH-.
The hydrogen ion, is not just a bare proton, it is a proton bonded to a water molecule, H2O. This results in a hydronium ion, H3O+.
In Arrhenius's thoery, something that is a strong acid is a substance that completely ionizes in aqueous solution to give a hydronium ion, H3O+, and an anion. An anion is a negatively charged ion. An example of a strong acid is perchloric acid:
What is going on above is that we have perchloric acid in an aqueous soluion. This perchloric acid ionizes entirely and results in an hydronium ion and a perchlorate anion. Some other examples of strong acids would be: HI, HBr, HCl, HNO3, and H2SO4.
Now on to bases...A strong base is something that completely ionizes in aqueous solution to give a hydroxide ion and a cation. A cation is a positively charged ion. Strong bases are most of the hydroxides of Group IA elements and Group IIA elements including LiOH, NaOH, KOH, Ca(OH)2, Sr(OH)2 and Ba(OH)2.
Many of the acids and bases that we
encounter in our everyday lives are not strong acids, they are considered
weak. Weak acids do not completely ionize in solution, but exist in
equilibrium. Let's look at the reaction for acetic acid:
Information provided by: http://library.thinkquest.org