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I'm attempting to siphon boiling water into another container for a school project. I have researched the physics of a siphon but I can't seem to make my boiling water siphon out of its initial pot. Does the pot need to be airtight to increase the pressure? Is 3/8" tubing too thick for this to work?

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When you say "boiling" do you mean "I just took it off the stove so it's pretty hot", "it is on the stove and more or less simmering" or "it's on the stove and has a really good boil going". I am envisioning some possible answer based on the thermodynamics on the phase change but I need to understand just how boiling it is. Also, how far up is the top of the siphon relative the source surface? –  dmckee Feb 6 '13 at 23:30
    
The water is on a burner so is is a constant boil. The top of the siphon goes 1 inch above the source surface. –  Tyler Feb 6 '13 at 23:42
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You don't have to boil it for a normal siphon to work. For simplicity just ignore the boiling and pretend it is normal water (just don't touch the water...). So do normal siphoning for it as you would for normal, kitten-friendly room-temperature water. But if you're trying to make a 'more interesting' siphon which uses the energy from boiling water to push water through the siphon, then just make the pot airtight and use a tube with small diameter and a container with large diameter (and minimize amount of initial air in the airtight container). –  raindrop Feb 7 '13 at 0:34
    
As Raindrop said, first make it work with room-temperature water. Then with hotter water. Then with boiling water. The only thing tricky is how to get it started. To do that, you could use flexible tubing immersed in the water. Fill the tube with water, and then pinch shut one end, lift that end out of the water and stick it in the other pot, making sure the pinched end is lower than the original water surface. Then un-pinch it, and it should run. –  Mike Dunlavey Feb 7 '13 at 1:15
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2 Answers

The pot does not need to be airtight and shouldn't be airtight. A siphon works due to air pressure on the surface of the water. Making it airtight would isolate it from the air pressure and you wouldn't be able to siphon the water.

I'm assuming that you know how to siphon non-boiling water? If you've never gotten a regular siphon to work get two containers and try siphoning the water back and forth before you move on to boiling water. The exit from the siphon tube must be lower than the surface of the water in the container being siphoned. The lower the exit the greater the pressure and the faster the water will flow. The siphon tube must be full of water from the container being siphoned to just past the point where it's lower than surface level of the water for the siphon to start.

If you can get the standpipe fitting from an old electric percolating coffee pot you should be able to stick that in the boiling water, attach your hose to the top of that, and the boiling water will actually prime and start the siphon for you.

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"Making it airtight would isolate it from the air pressure and you wouldn't be able to siphon the water." If the container of boiling water is airtight and maintains a constant volume, then the pressure of air in that container will become bigger due to the steam. So making the container airtight will increse the air pressure. But of course siphoning off the water would have the effect of increasing the volume and thus decreasing the pressure in the container. Personally I prefer not making it airtight (there's a risk of explosion/cracking) –  raindrop Feb 7 '13 at 0:26
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I'm thinking if the water is truly boiling, it will boil in the siphon and form steam, thus destroying the siphon. So maybe it's impossible to siphon boiling water.

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This is the direction I was going. Especially if the peak of the siphon is significantly above the surface causing a drop in pressure and driving a significant amount of mass to change to vapor. That said, it is not clear to me that I know how to analyze this idea. –  dmckee Feb 7 '13 at 1:10
    
Yes the decreased pressure will lower the boiling point, making the water super heated, and thus a certain percent should flash into steam –  Jim Graber Feb 7 '13 at 7:52
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