when helium gas is filled in a balloon,it does not rise after a certain height.Why?

Answer 1

After a certain height, because the air pressure outside the balloon is so low, the balloon stretches so much that it bursts.

If you could make the balloon out of something that didn't burst, then it would also not stretch....if it stopped stretching, then the balloon would rise to the point where the density of the gas inside the balloon and air outside the balloon would be close enough that the balloon stops rising....i.e., the densities would be essentially the same.

If you could make the balloon out of something that stretched without breaking for ever.... then it would rise to the point that the density outside the balloon is just above zero and the density difference or buoyancy is just strong enough to balance out the force of gravity.

Answer 2

The higher the balloon goes the less pressure there is to force the balloon upward