Well, I'm not sure.
When you heat the gas inside the balloon, the pressure
inside it increases. So you would think right away that the
balloon would get bigger.
BUT ... the pressure under water is greater than the pressure was
in air. So if the pressure inside the balloon doesn't increase enough
to compensate for the increased pressure outside of it, then it will
actually get smaller.
I guess it depends on HOW hot it gets, and on HOW deep you
submerge it.
Thinking about this gives me an idea for a neat experiment that I might
try some time:
Just take an small inflated balloon to someplace where you have a
substantial depth of water available, like to the lake or a swimming pool.
Then submerge it. The pressure under water changes pretty fast as you
go deeper ... at 33 feet, it's double the normal atmospheric pressure.
So maybe, as you move the balloon up and down under water, you might
be able to actually see it get bigger and smaller !
You don't want to use too big of a balloon. If it's only 1 cubic foot,
then in order to submerge it completely, you have to pull it down
with 63 pounds of force ! To submerge a bigger balloon, you'd
need to pull it down even harder.
