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 appears that the water isn't 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 ?

  • $\begingroup$ 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. $\endgroup$
    – Hoytman
    Mar 31, 2014 at 21:18

1 Answer 1


Water boils when the pressure is less than its 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.

  • $\begingroup$ 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 ? $\endgroup$ Apr 3, 2013 at 23:18
  • $\begingroup$ 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. $\endgroup$ Apr 4, 2013 at 5:42
  • $\begingroup$ 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. $\endgroup$ Apr 4, 2013 at 6:58
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    $\begingroup$ Using an example other than water, butane is kept liquid in lighters because the vessel can exert a pressure greater than the vapour pressure of the liquid (which otherwise boils at atmospheric pressure, room temperature). The same for aerosol cans, propane tanks, etc. $\endgroup$
    – J...
    Aug 29, 2014 at 10:29
  • $\begingroup$ @JohnRennie I think in a huge vacuum chamber, the reason the water would start to boil but not finish is because the boiling cools the water lowering the vapour pressure. Balloons exert less pressure as they get bigger and a gas takes up so much more space than a liquid so it would not be until the balloon was really huge with a significant fraction of the water turned into steam that it stops growing. If there's a tiny air bubble in the balloon, it might grow until the partial pressure of the air is low enough and the bubble is still much smaller than the balloon. $\endgroup$
    – Timothy
    May 15, 2018 at 3:24

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