Science Questions

If you pop a balloon in space, would the vaccuum cause the gas to expand at an ever increasing rate?

Tue, 4th Oct 2016

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Staffan Lincoln asked:

If you pop a balloon in space, would the vaccuum cause the gas to expand at an ever increasing rate?


/Staffan Lincoln




We put Staffan's question to Caroline Steel... A balloon

Caroline - So firstly, unfortunately, you couldn’t pop a balloon in space because as soon as you got the balloon into space it would split. And that’s because, as the questioner said, there’s more gas inside the balloon and there’s less gas outside the balloon in the vacuum of space. So the balloon would expand as the gas inside it would want to move towards the area of  no gas. So if you could video it and play it in slow mo you could see the balloon expand and then probably split down one side.

Unfortunately, you wouldn’t even be able to hear this sort of ‘pop’ as there’s no molecules to take the sound to your ears so you wouldn’t hear it. And then once the balloon has split the molecules from inside the balloon would travel off into the vastness of space never to be seen again.



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The first thing to say is that a vacuum doesn't suck - it is the pressure of the gas in the balloon that blows.

Inside the intact balloon, with 1 atmosphere of pressure, the air molecules travel at an average speed around the speed of sound, 340m/s. The mean free path before they bump into another molecule (or the balloon) is about 70nm.

When you pop the balloon, the molecules which would otherwise have hit the plastic at a speed of around 340m/s will just continue into empty vacuum. Some molecules will happen to be travelling a bit faster than the average (say, 500m/s) and will overtake or bump into molecules that were closer to the plastic.

By the time the cloud of gas has expanded to 1000x the diameter of the balloon (in around 1 second), the density has reduced by a factor of a billion, and the molecules will now travel meters before they bump into another molecule. If they don't bump into another molecule, their speed will not change.

Overall, the gas molecules are slightly attracted to each other by electrostatic forces. It takes energy to increase the average separation, and this energy comes from their velocity. The velocity of the molecules in the expanding gas cloud is slightly lower than the velocity before the balloon was popped. This is apparent in a decline in the temperature of the gas as it expands. 

The velocity will not continue to increase - it won't even reach the velocity it had before the balloon was popped.


evan_au, Fri, 23rd Sep 2016

It's also worth noting that, while it is true that the molecules will slow down, they won't slow down much. The attractive forces are small and they fall off very quickly as the molecules get further apart. Bored chemist, Fri, 23rd Sep 2016

Thanks evan_au. Your answer completely changed my mental model of gas and vacuum. I thought of gas and vacuum as two variants of filled space. One with high density and one with 0 density. I also thought of the air as being completely still within the balloon. And since I've seen lots of science fiction scenes where the air rushes out of a breach in the hull of a space ship, I thought it was the low pressure that somehow pulled the air out of the ship. Since space is largely empty, I imagined this pulling effect would perhaps account for the increased rate of expansion of the universe. From your answer, I now imagine the air molecules as lots of rapidly moving pool balls, who are trapped by a rubber barrier. Staffan Lincoln, Wed, 5th Oct 2016

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