[Physics] A water balloon in vacuum: does it boil


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 ?

Best Answer

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.

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