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Q & A: closed capillary tube

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Most recent answer: 08/22/2015
Q:
How can capillary action work as you show it when the tube is CLOSED at the top?Water moving up the tube would have to compress the air above it and the weak hydrogen bonding between the water molecules cannot overcome the pressure of the compressed air above it in a closed space. As the volume of the air in the cloesed tube is compressed and becomes smaller, the air pressure must increase accordind to Boyle's Law so equilibrium between the capilary action and volume loss would occur with very little compression and very little water in the tube.The tube should be drawn so it is open at the top and not closed!
- Henry (age 72)
United States
A:

You are totally right in observing that there will be some air trapped within the tube if you simply dip an inverted tube into water. It is also absolutely correct that as the water level rises, there will be some compression of the air, hence an increase in its pressure. My answer is, that system can do some work against the gas compression, because water and the surface are not totally inert.  Water molecules can adhere to hydrophilic surfaces, which will be releasing free energy hence constituting the required driving force against gravity and air pressure so water level will slightly rise. The extent of this effect depends on how you initially set the setup, of course. You could do:

1. Fill a long closed tube with water and then invert it. In this case, there will not be air inside (ideally), but a quite strong air pressure outside, so water level will be very high (~10m). On top of this, there will be a smaller contribution from capillary action. Better choose a long pipe! 

2. Use a tube with open ends, in which case water level will rise slightly only due to capillary action. Atmospheric pressure will cancel out on both sides. The extent of rise will be the same as in capillary action contribution in case 1.

3. Dip an empty tube with closed end into water, trapping some air inside. Due to the counteracting air pressure, there will be some rise in water due to capillary action but less than in case 2. But the equilibrium level will be different than the initial level.

Tunc


(published on 08/22/2015)

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