what will happen to a balloon stretched over the mouth of a bottle when steel wool inside the bottle rusts?

Answer 1

The guy above is sharp, and I'm pretty sure he's right.

Answer 2

In real life, nothing will happen to the balloon because the balloon is slightly permeable, and air can pass through it, although slowly. Your birthday balloon will lose air faster than a chunk of steel wool in a jar will rust. Sorry to ruin your experiment.

But since rusting takes oxygen, and converts the gas into a solid, then a fixed volume of gas in the jar would shrink. The volume increase of the rust is negligible compared to the volume decrease of the gas.

That was a fun question, thanks.