[lfo98]

CliffordK the heights you have quoted are the theoretical predictions based on a siphon acting under atmospheric pressure. This height is proportional to the magnitude of atmospheric pressure, in the experiment I'm sure the pressure is reduced enough to make this theoretical height miniscule (much less than the height achieved in the experiment).

What is the maximum height achievable with this liquid is a good question (and how is this dependant upon air pressure), but I don't think you're predictions based on density are necessarily going to be correct.

Also nice point about vapor pressure, I can only assume that air pressure is effectively reduced with the increase of vapor pressure (either by decreasing air pressure or heating the liquid). The liquid in the video however has no significant vapour pressure, apparently it will sit in space for years.

[CliffordK]

I'm sure the ionic liquid would be odd to work with. 

If you take a Mercury barometer, the space above the liquid is essentially a pure vacuum (plus some Mercury vapor due to the vapor pressure).  Lowering the pressure, and the mercury level gets lower.  Raising the pressure and the mercury level goes up.  However, this vacuum gap at the top would be sufficient to break a siphon.

If one made a column of the ionic liquid, one would likewise eventually reach a height where a vacuum gap would form.  And, if it is part of a siphon, the vacuum gap would break the siphon.  And, this would be dependent on the pressure.

However, it might be somewhat like a supercooled liquid, in that the level may raise up beyond the expected maximum, then once the gap starts to form, it would quickly drop down to the expected height.

Unlike the Mercury, the gap above the ionic liquid would be essentially a pure vacuum without vapor from the liquid.

[Commodity Tips]

Touchee.
I wrote that badly. The air pressure on the upper surface is higher than the air pressure at the top of the U. That is enough to keep the top of the U full of water. (There is a limit of about 10m to the height to which atmospheric pressure will push the water up and over. In practice, this limit is quite a bit less than 10m)

The pressure difference between top and bottom of the down pipe keeps the  water flowing out of the bottom. The greater the 'drop' the faster the flow of water. (Think of old fashioned toilet cisterns put near the ceiling.)

Is that better, Chris?

Because of gravity, water and most liquids seek their own level. This means that if left side by side, gravity would not be able to move the water in any direction. But if you were to move one container lower than the other then water could move in the direction of the lower level container through a tube or pipe that was attached to the bottom of the higher container. But if the tube used to move the water had to be raised higher than the upper container, suction would get the water moving in the direction of the lower container and then gravity would take over and the water would continue to move without further suction. A siphon doesn't "defy" gravity to work, but uses gravity to perform the siphoning action.

[imatfaal]

Touchee.
I wrote that badly. The air pressure on the upper surface is higher than the air pressure at the top of the U. That is enough to keep the top of the U full of water. (There is a limit of about 10m to the height to which atmospheric pressure will push the water up and over. In practice, this limit is quite a bit less than 10m)

The pressure difference between top and bottom of the down pipe keeps the  water flowing out of the bottom. The greater the 'drop' the faster the flow of water. (Think of old fashioned toilet cisterns put near the ceiling.)

Is that better, Chris?

Because of gravity, water and most liquids seek their own level. This means that if left side by side, gravity would not be able to move the water in any direction. But if you were to move one container lower than the other then water could move in the direction of the lower level container through a tube or pipe that was attached to the bottom of the higher container. But if the tube used to move the water had to be raised higher than the upper container, suction would get the water moving in the direction of the lower container and then gravity would take over and the water would continue to move without further suction. A siphon doesn't "defy" gravity to work, but uses gravity to perform the siphoning action.

Commodity Tip - please reference anything copied and pasted from another site.  Your answer is entirely from
http://wiki.answers.com/Q/How_does_a_siphon_work