Aluminum Hydroxide - Properties

Aluminum hydroxide is found in nature as the mineral bayerite or gibbsite (also called hydrargillite). A mixed aluminum oxide-hydroxide mineral is known as diaspore or boehmite.

In a purified form, aluminum hydroxide is either a white bulky powder or granules with a density of about 2.42 g/mL. It is insoluble in water, but soluble in strong acids and bases. In water, aluminum hydroxide behaves as an amphoteric substance. That is, it acts as an acid in the presence of a strong base and as a base in the presence of a strong acid. This behavior can be represented by the following somewhat oversimplified equation.

In the presence of a strong acid such as hydrochloric acid, the above equilibrium shifts to the right, and aluminum chloride is formed.

In the presence of a strong base such as sodium hydroxide, the equilibrium is driven to the left and a salt of the aluminate ion (AlO_2-) is formed.

Sodium aluminate, NaAlO₂, has a number of practical applications, such as in water softening, the sizing of paper, the manufacture of soap and milk glass, and in the printing of textiles and fabrics.