A chemical change by which ions are formed from a neutral molecule of an inorganic solid, liquid, or gas. The most common type of ionization occurs when an ionically bonded inorganic compound such as sodium chloride or sulfuric acid is dissolved in water (or other solvent); the molecule separates or dissociates into two ions, the metallic ion being positively charged by loss of an electron and the nonmetallic ion being negatively charged by gaining an electron. The degree of dissociation varies with the type of compound, the solvent, and the temperature. Molecules or atoms of gases are ionized by passage of an electric current through the gas; this removes electrons and leaves a positive charge. Compounds that ionize in solution greatly increase the conductivity of the solvent. Ionization is most effective in water because its high dielectric constant lowers the ionic bonding forces in the solute molecules enough to cause separation of their constituent atoms. Ion formation produces a notable rise in the boiling point and a depression of the freezing point of water. An electric current passed through a solution containing ions causes them to move to the oppositely charged electrode; this effect is the basis of many industrial electrochemical operations, such as electroplating and the manufacture of sodium hydroxide and chlorine.See Ion; Electroplating; Electrolysis; Electrolyte; Dissociation.
