The individual dihydroxybenzene solutions are mixed with aqueous FeCl₃. 4 mL of FeCl₃ solution are poured into the first conical measure, 10 mL into the second measure and 20 mL into the third measure.
Results:
When treated with aqueous FeCl₃, the aqueous dihydroxybenzene solutions will show the characteristic color change.

Discussion:

Like phenol also catechol and resorcinol do form a colored complex with FeCl₃.

Hydroquinone is oxidized rapidly to p-benzoquinone, which does not generate a colored complex with FeCl₃.

Redox equilibrium between hydroquinone and p-benzoquinone:

Resorcinol shows no redox reaction with Fe³⁺. The two hydroxy groups in the meta position can not form a quinoid system. Thus a redox reaction between resorcinol and Fe³⁺ is impeded.

Catechol is only partially oxidized to o-benzoquinone.