`SnO_(2)` forms positively charged colloidal sol in acidic medium and negatively charged colloidal sol in basic medium. Explain ?

Explanation

`SnO_(2)` is amphoteric reacting with acid and base both. In acidic medium (say HCl) `Sn^(4+)` ion is formed which is preferentially adsorbed on `SnO_(2)` giving positively charged colloidal sol :
`SnO_(2) + 4HCl to SnCl_(4) + 2H_(2)O ,`
`SnO_(2) + SnCl_(4) to [SnO_(2)]Sn^(4+)` (Positively charged) `+ 4Cl^(-)`.