Pure water at 25°C ionizes as what?

The ionization of pure water is an ionization reaction in which a water molecule loses the nucleus of one of its hydrogen atoms and becomes a hydroxide ion. The ionic product of pure water is denoted as Kw. During this process hydronium is produced (H3O+). At 25 degrees the Kw is 13.99.

Pure water molecules are composed of two hydrogen atoms and a single oxygen atom. Water molecules can decompose quite readily around room temperature into their constituent ions. Pure water at 25 degrees Celsius ionizes into positive hydrogen ions (H+) and negative hydroxide ions (OH-). This water molecule is said to deprotonate.