Both the compounds potassium hydroxide (KOH) and sodium hydroxide (NaOH) have ions, but when they are in a solid state, the ions are not free here. The moment when they are dissolved, they become fully ionized and produce free hydroxide ions. Only hydroxide ion in the solution carries a negative charge.
Ammonia gas is the summation of ammonia molecules. When ammonia gas is dissolved in water, their reaction will produce ammonium ion and hydroxide ion. However, ammonia dissolves only partially in water and very small number of hydroxide ion is produced. Therefore, ammonia solution contains ammonia molecules, water molecules and a small number of ammonium and hydroxide ions.
The characteristic of an alkali solution depends on the presence of mobile hydroxide ion. The alkalis that dissolve partially in aqueous solution are called weak alkali. Strong alkali becomes fully ionized in aqueous solution i.e the weak alkali solution consists of a lesser number of hydroxide ion than that of the strong alkali.