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Q & A: Boiling water in syringe

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Most recent answer: 01/31/2011
Q:
If we boil water in a syringe and could maintain a vacuum how long would it boil? And why doesn't the hand holding the syringe feel colder?
- Ed Boerema (age 61)
Montague Mi.
A:
You're right that when you boil water in a syringe by pulling back on the plunger the water should cool down some. That's because boiling soaks up some latent heat. Probably you don't feel it because you can't boil much water this way, so you can only cool it a little bit. The reason is that the vapor from the little bit of boiled water quickly raises the pressure in the syringe up to the vapor pressure of water at the water's temperature, so the boiling stops.

To "maintain a vacuum" you'd need a giant syringe, compared to the volume of liquid water, since the vapor is so much less dense than the liquid. In practice, when someone wants to boil away the water using a vacuum, one uses a flask with an opening connected to a vacuum pump.

Mike W.

(published on 01/31/2011)

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