The extent to which a species in solutionA mixture of one or more substances dissolved in a solvent to give a homogeneous mixture. ionizes. Often applied to the ionizationA process in which an atom, molecule, or negative ion loses an electron; a process in which a covalent molecule reacts with a solvent to form positive and negative ions; for example, a weak acid reacting with water to form its conjugate base (an anion) and a hydrogen (hydronium) ion. reactions of acids and bases.