Ammonia has the chemical formula NH3. It is a compound of hydrogen and nitrogen. Ammonia is a colourless gas with a strong, pungent odour. It is soluble in water and forms a weak base called ammonium hydroxide.

Ammonia is a base because it can accept protons (H+ ions). In water, ammonia reacts with water to form ammonium hydroxide:

NH3 + H2O → NH4+ + OH-

The ammonium ion (NH4+) is a weak acid, so it can donate protons to water to form hydronium ions (H3O+):

NH4+ + H2O → H3O+ + NH3

The equilibrium constant for this reaction is Ka = 1.8 × 10-5, which means that ammonia is a weak base.

The pH of a solution of ammonia is determined by the concentration of ammonium hydroxide. At 25 °C, a 0.1 M solution of ammonia has a pH of 9.25.