ammonia, chemical compound, NH3, colorless gas that is about one half as dense as air at ordinary temperatures and pressures. It has a characteristic pungent, penetrating odor. Ammonia forms a minute proportion of the atmosphere; it is found in volcanic gases and as a product of decomposition of animal and vegetable matter. Because ammonia was formerly obtained by destructive distillation of horns and hooves of animals, its water solution was called spirits of hartshorn. Ammonia has also been called alkaline air and volatile alkali.
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