Acid and alkali

Question

for instance, NH3 ammonia solution, why there's no OH- but it is still an alkali? Is it because the compound is in negative charge? If so, is it all compound anions are alkali or bases?

Answer 1

Pure ammonia (NH3) is a neutral compound, which is a gas at room temperature and pressure.

Ammonia solution is formed by dissolving ammonia in water. Ammonia solution is alkaline, because ammonia is ionized in water to give hydroxide ions (OH-(aq)).
NH3(aq) + H2O(l) = NH4+(aq) + OH-(aq)
(a reversible reaction)

Answer 2

The NH3 will form NH4+ ion with water and release OH- ion while it is a solution.
To form acid, we need to have H+ more than OH-, we do NOT need to add H+ to form an acid, we can reduce the amount of OH-. This the same case, NH3 will reduce H+ ion and increase OH- to form an alkali.

2007-06-22 15:11:22 補充：
Since the number of H+ and OH- are almost the same in water, (~10^7 per litre) it is neutral.We can reduce the amount of OH- or increase amount of H+ to make it acidic and vice the verse.