If FeCl3 is added to excess of hot water, a postively charged sol of hydrated ferric oxide is formed due to adsorption of Fe3+ ions. However, when ferric chloride is added to NaOH, a negatively charged sol is obtained with adsorption of OH- ions. Why is there such a difference? Explain in detail.

 A colloidal particle usually adsorbs those ions which are in excess and are common to its own lattice. This preferential adsorption of a particular type of ions imparts a particular type of charge to colloidal particles.
When FeCl3 is added to water it will prepare a Fe(OH)3 sol where Fe3+ ions are present in excess in dispersion medium and are common to lattice of Sol so it get preferentially adsorb on sol give the sol overall positive charge. 
when FeCl3 is added in NaOH, again the constituent of the sol is same that is Fe(OH)3 but now the dispersion medium is excess with OH- ion hence it get preferentially adsorb to the sol giving overall -ve charge.
 

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