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If I put water in a vacuum it will boil. But what if I put this water inside a balloon ? I searched for answers and fount this video: http://www.youtube.com/watch?v=9q8F3ClUuV0

It seams that the water is not boiling. But I am still not satisfied. The vacuum on the video could be just not enough to make the water boil at its temperature. Or it might be because of the pressure of the balloon.

If I heat up a water balloon over 100°C at atmospheric pressure, it will boil and grow. So why not at 20°C in a vacuum ?

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Keep in mind, the decompression you would experience by being in a vacuum is not a strong as the pressure you would experience by being under 20 feet of water. –  Hoytman Mar 31 at 21:18
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Water boils when the pressure is less than it's vapour pressure (there is a table of vapour pressure vs temperature here).

At 20ºC the vapour pressure is 2339Pa, so if your balloon exerts a pressure greater than this the water won't boil. If the pressure exerted by the ballon is less than this, the water will start to boil and the steam generated will inflate the balloon. This will increase the pressure until the pressure has risen to 2339Pa, and at this point the water will stop boiling and you're left with water and steam in equilibrium.

If you heat water to 100ºC the vapour pressure rises to 1 atm (101325Pa), which is of course why water boils at 100ºC at sea level. However if you have a really strong balloon capable of exerting a pressure of greater than 1 atm the water won't boil at 100ºC even in a vacuum, and you'd need to raise the temperature above 100ºC to make it boil.

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So you suggest that in the video I linked, the pressure in the jar is greatter than 2339Pa (assuming temperature is 20°C). The additional pressure exerted by the balloon seams to be negligeable. Do you agree ? –  Tom Esterez Apr 3 '13 at 23:18
    
No, I think the additional pressure of the balloon is significant. 2339Pa is only 1/50 of an atmosphere and I think the balloon is easily capable of exerting that pressure even though it's only partially inflated. –  John Rennie Apr 4 '13 at 5:42
    
Actually there may be another reason for the water not boiling. Creating bubbles of steam requires energy because there is a barrier to bubble nucleation. It may be that the water in the balloon is superheated. The inside of the balloon is probably a low energy surface and if the water is reasonably pure it could probably be superheated quite easily. –  John Rennie Apr 4 '13 at 6:58
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