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My 9yr old is inspired to try to make ice with a vacuum after seeing an exhibit do just that at room temperature at the Exploratorium in San Francisco.

He's using a motorcycle vacuum brake bleed pump that pulls a 760mmhg vacuum, and we're putting water into a plastic container in that system and pulling the vacuum on that, it's not working, any ideas what we need to do to get this working? Is the vacuum strong enough? Would the water need to be cooled to begin with? We're not sure if the water in the Exploratorium exhibit was 'pre-chilled' to a specific temperature or not.

However it sounds like from other posts here the pressure we're able to get from the vacuum pump we're using should be enough to get the water to hit its solid/liquid/vapor triple point which should be enough to get it to freeze. Any ideas how to tweak this experiment to make it a success?

Agent Zebra
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    I did this in high school with room temperature water and 2 aspirators, so your pump should be fine. Are you sure you don't have any leaks? Is your plastic container strong enough, or is it collapsing a little? Are you getting vigorous boiling? I used a thick-walled glass flask, perhaps your container needs better insulation so the water isn't kept warm by the ambient temperature. – PM 2Ring Jun 11 '18 at 00:39
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    Re, "pump that pulls a 760mmhg vacuum." Did you measure that? There's a difference between a spec sheet that says the pump can achieve 760mm when pumping the air out of an empty chamber, and being able to achieve an actual 760mm when there's water evaporating inside the chamber. – Solomon Slow Jun 11 '18 at 00:50
  • @jameslarge measuring with a vacuum gauge – Agent Zebra Jun 12 '18 at 18:18

1 Answers1

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Three suggestions:

  • a small amount of water: you have to evaporate a large fraction to freeze it, and a large fraction of a lot of water is a lot of pumping

  • start with cold water: the colder the water, the higher the freezing pressure

  • keep it well insulated: energy flowing in the through the plastic container will warm it up

  • keep pumping: you have to evaporate about 15% of the water to freeze the rest

There’s more on this here.

Bob Jacobsen
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