The following chemical equilibrium exists in ammonia water:

NH3+H2O=(reversible)=NH3 · H2O

NH3 · H2O=(reversible)=NH4++OH-

Therefore, only a small portion of ammonia molecules react with water to form ammonium ions NH4+and hydroxide ions OH -, resulting in weak alkalinity.

Ammonia water has alkaline properties:

① Can turn colorless phenolphthalein test solution red, purple litmus test solution blue, and wet red litmus paper blue. This method is commonly used in the laboratory to detect the presence of NH3.

② Can react with acids to generate ammonium salts. When concentrated ammonia water meets volatile acids such as concentrated hydrochloric acid and concentrated nitric acid, it produces white smoke.

NH3+HCl=NH4Cl (white smoke)

NH3+HNO3=NH4NO3 (white smoke)

However, this phenomenon does not occur when encountering non-volatile acids such as sulfuric acid and phosphoric acid. This method can be used in the laboratory to detect the presence of NH3 or ammonia water.

In industry, the weak alkalinity of ammonia water is used to absorb sulfuric acid industrial exhaust gas and prevent environmental pollution.

SO2+2NH3 · H2O=(NH4) 2SO3+H2O

(NH4) 2SO3+SO2+H2O=2N4HSO3