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I've been arguing with a friend about the whole "perfect vacuum" concept. He and I agree that the most powerful vacuum pump in the world couldn't pump water more than ~34 feet above the surface of a regular elevation lake. I changed the problem slightly, and I'm not sure what would happen.

  1. Submerge a hose such that it was completely filled with water
  2. Cap off the top end
  3. Start pulling it out of the water by a crane
  4. Make sure the bottom end stays submerged

If you pulled the hose 40 feet out of the water, what would be in the top 6 feet of the hose? A "perfect vacuum"? Gaseous water? Liquid water?

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  • $\begingroup$ The theoretical pumping limit for a suction pump is approximately thirty-four feet at sea level, and less at higher altitudes (about 1 foot less per 1000 feet). That is the limit regardless of the motor size connected to the pump because suction is limited to one atmosphere of negative pressure. Over 34 feet you would be pulling a perfect vacuum. Reference: windsun.com/Solar_Water_Pumps/pumpinfo1.htm $\endgroup$ – Argus Jun 9 '12 at 4:38
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A low pressure water vapour.

Not quite a perfect vacuum because at low pressure the boiling point of water is reduced and so some of the water molecules escape into the empty space.

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Cohesion tension would keep the water arc together as it does in the transpiration process within Redwood trees. If, on the other hand, you allowed the water column to separate, you would not be able to pump it to the 35-40' top of the arc.

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The water will flash to steam, filling whatever void is at the top -- however high. Boiling is a product of temperature AND pressure. 212F (100C) at 15psi (atmosphere) is what most people understand, but at a lower pressure, water boils at a lower temperature. At some lower pressure, water goes straight from solid (ice) to gas (steam).

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