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# Why is a negatively charged sol formed when FeCl$_3$ is added to NaOH?

$\begin{array}{1 1} \text{(A) due to adsorption of OH$^{-}$ions} \\ \text{(B) due to the presence of a basic group} \\ \text{(C) due to the adsorption of Fe$^{3+}$ions} \\ \text{(D) self-dissociation} \end{array}$

It has been found that sols are often associated with small quantities of electrolytes. If the electrolyte (ions) is completely removed from the colloidal solution by repeated dialysis or ultra filtration, the sol becomes unstable. Thus, it is believed that the charge on the colloidal particles is due to the preferential adsorption of either positive or negative ions on their surface.
A negatively charged sol of hydrated ferric oxide is formed when ferric chloride is added to NaOH solution as follows:
FeCl$_3$ + 2NaOH $\rightarrow$ Fe(OH)$_3$ + 3NaCl.
The neagtively charged sol is obtained because of the preferential adsorption of the OH$^{-}$ ions which form an electrical double layer.