Question

A balloon is negatively charged by rubbing and then clings to a wall. Does this mean that the wall is positively charged ? why does the balloon eventually fall?

Answer 1

No, it does not necessarily means that the wall was initially positively charged. The molecules in the wall can be polarized due to the negative charges on the balloon and can attract the negatively charged balloon.

The excess electrons ( negative charges ) in the balloon will be slowly discharged through the wall ( since it is not a perfect insulator ) and as the balloon becomes neutral, it will fall.