Demos: 2A-07 Fluid Transfer Under Low Pressure

A flask, partially filled with colored water, is tightly sealed with a stopper through which passes a U-shaped glass tube. The other end of the tube is inserted into a beaker. The entire system is placed under the bell jar of a vacuum pump. The slightest evacuation of air from the bell jar causes a transfer of the water from the flask to the beaker. When the air is let back into the bell jar, water flows back to the flask.

Directions: Carefully place the entire system under the bell jar, making sure the O-ring is in place. Close the valve and turn on the vacuum pump. Immediately the water begins to transfer to the beaker, continuing until the level drops below the bottom of the glass tube. After explaining what has happened, SLOWLY open the valve and the liquid will transfer back to the flask.

Suggestions for Presentation: It is helpful to have a drawing of the situation to use on the overhead projector. Carefully explain how the pressure will be greater inside the flask when the outside is partially evacuated, resulting in a transfer of fluid. Then ask what will happen if you let the air back in the bell jar. (As an extra consideration: Work is done in lifting the water back and forth in the system. What is the source of this work?)

Applications: None that are evident.

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