Question

A balloon is filled up completely with helium gas and left in a room with a controlled temperature of 230C. After two days of being in the same room, the balloon was found to have shrunk to half its filled-up size. However, there was no leakage of helium gas from the balloon during the two days. Explain what happened to the pressure inside the balloon due to the change in the size of the balloon. ?  Use Boyle's Law to explain your answer.

Answer 1

Helium is a noble gas, which means each helium atom has a full valence electron shell. Because helium atoms are stable on their own, they don't form chemical bonds with other atoms.

So, helium balloons are filled with lots of tiny helium atoms. Regular balloons are filled with air, which is mostly nitrogen and oxygen. Single nitrogen and oxygen atoms are already much larger and more massive than helium atoms, plus these atoms bond together to form N₂ and O₂ molecules. Since helium is much less massive than nitrogen and oxygen in air, helium balloons float. However, the smaller size also explains why helium balloons deflate so quickly.

The helium atoms are very tiny -- so tiny the random motion of the atoms eventually lets them find their way through the material of the balloon through a process called diffusion. Some helium even finds its way through the knot that ties off the balloon.

Neither helium nor air balloons deflate completely. At some point, the pressure of gases on both the inside and outside of the balloon becomes the same and the balloon reaches equilibrium.

Gases are still exchanged across the wall of the balloon, but it doesn't shrink any further.