Question

What happens to a helium-filled latex balloon released into the air? Does it expand or contract? Does it stop rising at some height?

Answer 1

In the Earth's atmosphere the pressure and density vary exponentially which means that they will decrease as we go up. Helium has very low density, so the balloon will rise up to the point that it's density becomes equal to the density of the surrounding air.

On the other hand the balloon will keep expanding just to make sure that the pressure outside is smaller. There is a small leak of He gas from the balloon and street some time the balloon will run out of gas.