Why is NH3 (Ammonia) a base?
The acid base theory proposed by the Bronsted-Lowry theory says that an acid is the molecule that donates hydrogen ion in water. A strong acid, donates the hydrogen ion readily, while the weak acid slowly loses it. According to this theory, a base is the molecule that donates hydroxyl ion in water. A strong base is the one that gives away the hydroxyl ion readily while the weak base loses the ion slowly. But, Ammonia does not behave as either acid or base according to the Bronsted-Lowry theory. As ammonia does not have extra hydroxyl ion to donate, it cannot be classified as a base as per the above theory.
There is another theory put forward to explain the acid-base properties. Acidic and basic features can also be described using Lewis theory. According to the definition given by Lewis, an acid is the one that accepts an electron pair. If it can do it readily and easily then it is a strong acid, otherwise weak acid. A base is the one that donates an electron pair. If it readily donates the electron pair, then it is a strong base, otherwise a weak base. Ammonia has a lone pair of electrons on the nitrogen atom and can donate this electron pair. Hence, ammonia is considered as a base. Normally, it is accepted that a strong acid give rise to a weak conjugate base and a weak base give rise to a strong conjugate acid. As ammonia can give rise to ammonium ion, which is a strong conjugate acid; ammonia is classified as a weak base.
When ammonia is dissolved in water, it forms ammonium hydroxide. Ammonium hydroxide has the tendency to lose hydroxyl ion and hence act as base. In general, acids react with bases to form salts. Even ammonia reacts with acids to form salts. This reaction occurs in the presence of water or moisture. Hence, ammonia is considered as a strong base. Ammonia forms ammonium chloride with hydrochloric acid, ammonium nitrate with nitric acid in the presence of moisture and not with dry ammonia.