Amphoterism
Amphoteric oxides, such as aluminium oxide, are soluble both in strongly acidic and in strongly base solutions :
In acid:`Al_2O_3(s)+6H_3O^(+)(aq)hArr2Al^(3+)(aq)+9H_2O(l)`
In base:`Al(OH)_3(s)+OH^(-)(aq)hArr Al(OH)_4^(-)(aq)`
Dissolution of `Al(OH)_3` in excess base is just a special case of the effect of complex-ion formation on solubility.`Al(OH)_3` dissolves because excess `OH^(-)` ions convert it to the soluble complex ion `Al(OH)_4^(-)`(aluminate ion)The effect of pH on the solubility of `Al(OH)_3` is shown in figure.

Other examples of amphoteric hydroxides include `Zn(OH)_2,Cr(OH)_3,Sn(OH)_2 and Pb(OH)_2`, which react with excess `OH^(-)` ions to form the soluble complex ion `Zn(OH)_4^(2-)`(zincate ion),`Cr(OH)_4^(-)`(chromite ion),`Sn(OH)_3^(-)` , `Fe(OH)_2 and Fe(OH)_3` , dissolve in strong acid but not in strong base.
Which of the following curves best represents the variation of solubility of ferrous hydroxide `Fe(OH)_2` with the concentration of `[H^(+)]` ions in the solution:

Options

A.
B.
C.
D.

Answer

A

Explanation

On increasing concentration of `[H^+]` ions the solubility of basic hydroxide `Fe(OH)_2` will increase.