The Arrhenius definition of a base is a substance that increases the concentration of hydroxide ion in water. Water soluble hydroxide such as sodium hydroxide, potassium hydroxide, and calcium hydroxide are among the most common bases. When sodium hydroxide is dissolved in water, it completely dissociates into sodium and hydroxide ions. Because aqueous sodium hydroxide exists entirely as ions in solution, it is referred to as a strong base. It is also of course a strong electrolyte. The ammonia molecule does not contain hydroxide ions. Nevertheless, it increases the hydroxide ion concentration in water upon dissolution and is therefore a base. When ammonia gas dissolves in water, it reacts with water to form the ammonium and hydroxide ions. However, ammonia is a weak base. The equilibrium forming the hydroxide and ammonium ions lies largely to the left. At any given time, most of the ammonia in solution exists as NH3 molecules, rather than as ammonium and hydroxide ions.
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