SnO2 forms a positively charged colloidal sol in acidic medium and a negatively charged sol in the basic medium. Why ?

SnO2 forms a positively charged colloidal sol in acidic medium and a negatively charged sol in the basic medium. Why ? 
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Answer :

Ans. SnO2 is amphoteric in nature. It reacts with acid eg. HCl to form SnCl4 in the solution. The common Sn4+ ions are adsorbed on the surface of SnO2 particles giving them a positive charge.

SnO2 reacts with a base eg. NaOH to form Sodium Stannate in the solution. The stannate ions are adsorbed on the surface of SnO2 particles giving them a negative charge. 
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