One of the things I’m taking away from the degassing article is that the stuff that’s coming out of our kitchen faucet isn’t exactly water. It’s a solution with water as the solvent and N[sub]2[/sub]/O[sub]2[/sub] as the solute, and the solution is fully saturated with N[sub]2[/sub]/O[sub]2[/sub], especially if the pot-heads have left the landlord’s aerator alone. When we put this solution into a column, the pressure at the top is reduced (and the article confirms this does indeed occur) which in turn reduces the solvents ability to hold the solute. The extra N[sub]2[/sub]/O[sub]2[/sub] is “expelled” from the solution and precipitates back into it’s gaseous phase. The cavity created is both N[sub]2[/sub]/O[sub]2[/sub] and water vapor which would have a higher pressure. Only the weight of a column of 10 m is needed to overcome the upward pointed pressure force. Whereas with pure water, the cavity only contains water vapor. We need a column of about 15 m for the weight to overcome the pressure force.
The liquid water surface is always kicking out water molecules into the cavity above, and water molecules are always being recovered by the liquid from this cavity. When these two actions are occurring at the same rate, we’ll find the cavity has a very specific pressure (it’s vapor pressure) at a specific temperature, for water this would be 23.3 mbars at 20ºC. If we raise the temperature, the vapor pressure will increase and I’m sure there’s tables widely published that given this value for a wide range on conditions. This is all fine and dandy until we reach the the boiling point of water. At this point things change, we can no longer (normally) raise the temperature until we’ve completed the phase change of the water, from a liquid into it’s vapor. Thus, the “vapor pressure” of water at 100ºC (1 atm) can be a wide range of values including the pressure of the water being completely vaporized.
I bring this up as there seems to be a bit of confusion about exposing liquid water to a perfect vacuum. Yes, the water will begin to boil off but the water vapor collects in the cavity and we no longer have a perfect vacuum. As more water boils off, the pressure increases and this raises the boiling point of the water. With enough liquid water available, eventually the boiling point will be equal to the ambient temperature. Any remaining liquid water will remain in it’s liquid phase (except those molecules leaving and being replaced by molecules returning).
Now proceeding with the question whether 100ºC water (@ 1 atm) would form a siphon, I’m going to say yes, if the water is in it’s liquid phase. The cohesion required is provided by the innate nature of liquids. I’m also going to speculate that water vapor at 100ºC (1 atm) would also siphon if and only if some outside agency provides the cohesion required. Say for example in an environment of Hydrogen gas. We achieve our cohesion from the higher density of water vapor, in that we can collect the water vapor in a container. In this situation we (maybe) could siphon the water vapor over a very very small humping point height.
Gases are fluids and should obey the rules governing fluids, including developing upward pressure forces in the two sides of a siphon tube.