And by how much would the balloon expand, if at all?
It would totally depend on the material that the balloon was made of (how it could withstand internal pressure) and where, in space, you did the experiment. The temperature would be determined by thermal equilibrium with the received Sunlight and radiated energy. If it ended up at 'room temperature (not impossible) then it could boil if the pressure in the balloon were around 0.02Atmospheres. See this wiki link for a list of vapour pressures at 'typical' Earth temperatures.
If the pressure were limited by the balloon envelope then there would be a mixture of vapour and water droplets (held together by molecular forces) in equilibrium when equilibrium is reached. This is what happens on Earth; clothes dry when the partial pressure of water vapour in the air is less than the vapour pressure of the water.
Hope this wasn't a homework question; I have more or less answered it for you.
Were you referring to this one?: Vapour pressure of water
I don't think you've answered the question, but you've certainly given good clues on how to solve it.
For some god knows why reason, I researched party balloon pressure just a couple of months ago, and came across the following research demonstration:
John Phillips, Chemistry teacher at Spring Street International School, Friday Harbor Washington.
Unfortunately, you have to wait until the very end of the video to see the data. [3 whole minutes! I thought I was going to die.]
Anyways, I'm not sure what the answers are, to the questions:
as I was researching something else.
And this problem looks like it involves both a bit of maths and logic.
I've done quite enough of that this morning, and don't want to tax my brain.
Thanks. I forgot to press the link button - durr!
But the answer still depends on the material of the balloon. It's a very possible school level experiment with a bell jar and a vacuum pump. The level of vacuum achievable this way is not what you would get in space but it's definitely low enough to make the point.
A beaker of luke warm water can easily be made to boil under these conditions so it's all down to the strength of the balloon envelope and whether it can maintain an internal pressure of 2% Atmospheric without bursting. A kevlar / carbon fibre balloon would sustain 1 Atmosphere so the water would just stay the same in that case. Water 'bomb' balloons are deliberately chosen to be pretty weak . . . . .
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