Question

why ammonia is a base

Answer 1

Ammonia itself obviously doesn't contain hydroxide ions, but it reacts with water to produce ammonium ions and hydroxide ions.

Answer 2

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Ammonia is a typical weak base. Ammonia itself obviously doesn't contain hydroxide ions, but it reacts with water to produce ammonium ions and hydroxide ions. However, the reaction is reversible, and at any one time about 99% of the ammonia is still present as ammonia molecules.

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