Sno2 forms positively charged colloidal solution in acid medium and negatively charged in basic medium (VSA)

SnO ₂ is amphoteric in nature. It reacts with acid eg. HCl to form SnCl₄ in the solution. The common Sn⁺⁴ 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.

​regards