How does ammonia form hydroxide ions when mixed with water?

When ammonia is mixed with water, it acts as a base by accepting a proton from a water molecule, which functions as an acid in this reaction. This process is represented by the equilibrium equation:12

$${\text{H}}_2\text{O}+{\text{NH}}_3\rightleftharpoons {\text{OH}}^{-}+{\text{NH}}_4^{+}$$

Through this proton transfer, the water molecule loses a hydrogen ion to become a hydroxide ion (${\text{OH}}^{-}$), while the ammonia molecule gains the proton to become an ammonium ion (${\text{NH}}_4^{+}$).13

It is a common misconception that this reaction produces a distinct compound called ammonium hydroxide (${\text{NH}}_4\text{OH}$). Modern chemistry clarifies that this species does not exist as a stable molecule; rather, the solution consists of ammonia and water molecules linked by hydrogen bonds, existing in equilibrium with the resulting ions. Because this equilibrium favors the reactants, a typical ammonia solution remains only weakly basic.13