Concentrated aqueous sodium hydroxide can separate a mixture of

Concentrated aqueous sodium hydroxide can separate a mixture of $Al^{3 +} and$Fe^{3 +}$<br/><br/>$Al^{3 +}$forms white gelatinous precipitate of$Al{\left(OH\right)}_{3} , which is soluble in excess of NaOH.

$Al{\left(OH\right)}_{3} + NaOH \to NaAl{O}_{2} + 2{H}_{2}O$

$Fe^{3 +}$ forms reddish brown precipitate of $Fe{\left(OH\right)}_{3}$ , which is insoluble in excess of NaOH.