Question

Why does a balloon put in a vacuum starts to expand

Answer 1

Answer:

The air inside a balloon is constantly exerting pressure against the inner surface of the balloon. Under normal, atmospheric pressure, the air outside (which surrounds the balloon) exerts pressure on the outer surface of the balloon which balances the forces, keeping the balloon from expanding.

If you reduce the pressure on the outside, the air pushing against the inner surface will expand it outwardly. The less pressure, the more it expands. If you drop the outside pressure to a near vacuum, it will expand until the balloon either pops, or the elastic force of the stretched balloon balances the air pressure inside.

Hope it helps!