Which metal hydroxides are amphoteric

We will begin with the familiar case of a hydroxide acting as a base. One of the most common and familiar examples of an amphoteric hydroxide is aluminum hydroxide, Al OH 3. From our solubility rules, we know that Al OH 3 is largely insoluble in neutral water; however, in a strongly acidic solution, the situation changes.

This is a classic acid-base neutralization reaction: the HCl completely protonates all three hydroxides per mole of Al OH 3 , yielding pure water and the salt AlCl 3. From what we know about the basic properties of hydroxides so far, this is exactly as we would expect—so how can a hydroxide act as an acid?

Here, aluminum hydroxide picks up an hydroxide ion out of solution, thereby acting as a Lewis acid. How is this possible? Consider the Lewis structure for Al OH 3. Hydroxides of alkali earth metals are much less soluble. In an acidic solution such as stomach juice, the following reaction takes place,.

As usual, we write the equilibrium equation so that we can write the concentration below the formula. If we do not know the concentration, we assume it to be a variable x. Not all metal hydroxides behave the same way - that is precipitate as hydroxide solids. The charged species are soluble in water. There is a gradual transition from basic oxides to acidic oxides from the lower left to the upper right in the periodic table. This reactivity can be used to separate different cations, such as zinc II , which dissolves in base, from manganese II , which does not dissolve in base.

Aluminium hydroxide is another amphoteric species:.