Solar balloon vs hydrogen or helium filled balloon This is my very first post here and I used the search engine to make sure this set of questions was not asked before. Sorry if it is the case for some reason. Feel free to delete my post.
Here are my questions : 


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*Do helium or hydrogen filled balloons necessarily pop when they reach a certain altitude or do some make it back to the ground in one piece and why?

*Can you confirm that solar balloons (air) are the only type of balloons that can make it back to the ground in one piece?
Thanks in advance.
Chris
 A: Helium and hydrogen balloons don't inevitably burst as they rise, but they are frequently designed to burst at a certain altitude to return their payload to Earth.
The atmospheric pressure decreases with altitude, and for a fixed mass of gas the volume is (approximately) related to pressure by the ideal gas law:
$$ V = \frac{nRT}{P} $$
So as the pressure goes down the volume goes up. In a balloon things are a bit more complicated than this because the internal pressure is equal to the atmospheric pressure + the pressure increase caused by the tension in the balloon skin. And also the temperature $T$ changes with altitude so we can't just treat $T$ in the equation as a constant.
However the end result is that as a balloon rises its volume increases and stretches the balloon skin. If there is enough gas in the balloon, and if the skin is weak enough, the skin will stretch so far that it ruptures and the balloon bursts.
But there is no reason why you can't make the skin strong enough that it won't burst. You could also only partially fill the balloon. Both of these are likely to reduce the lift - a strong skin is likely to weigh more, and putting less helium into the ballon reduces the upthrust. However in principle there is no reason why such a balloon could not be built.
The solar balloons that you mention are open at the bottom so the pressure inside them is always the same as the pressure of the air around them. That's why this type of balloon does not burst. However these balloons have less lift than a (sealed) helium or hydrogen balloon because heated air has a much higher density than helium or hydrogen.
