Squeeze Bottle

Concepts – gases, liquids, air pressure, displacement, density, gravity

Materials – Two 1-liter bottles, Two #3 rubber stoppers with a hole in the middle, barrel of a ballpoint pen, water

Getting ready – the barrel of a ballpoint pen needs to be one that does not have a hole in it. The BIC® Round Stic Grip™ Ballpoint Pen works well. Remove all the inner parts from the pen; you only want to use the hollow pen barrel. If the #3 rubber stoppers do not have holes in them or need larger holes use a rubber stopper borer to create them. Then push a rubber stopper through both ends of the barrel until about 1 cm of the barrel shows above the stoppers.

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Figure 26: a pen barrel pushed into two #3 rubber stoppers.

Procedure – Fill one of the 1-liter bottles with water until it is about ¾ of the way full. Attach the stopper-barrel-stopper connecting device created in the Getting Ready section to the 1-liter bottle. Put the empty 1-liter bottle on top and carefully turn the contraption upside down so that you have a 1-liter bottle with air-connecting device-1liter bottle with water configuration. Initially no water or very little water will fall from the top 1-liter to the bottom 1-liter. Ask students what is holding the water up. Squeezing the bottom bottle with air will cause a larger displacement of water from the top bottle to the bottom one, causing bubbles to form at the bottom of the water which float to the top of the upper 1-liter bottle. Ask students why this happens.

Explanation – given the density of water and air one would expect the water to quickly flow from the top bottle to the bottom one. However, the air in the bottom bottle completely fills the space and does not permit the water in the top bottle to fall until some the air has been displaced into the top bottle. This demonstrates the fact that air takes up space (has volume) and that air can apply a force to another object. Gravity is the force that pulls the water down from one bottle to another. Gravity also pulls down on the air, but the upward force from the water (when it moves into the lower bottle) is greater than the force of gravity and it pushes the air into the top bottle.

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Figure 27: On the left no water (or very little) gets transferred from the top to the bottom. On the right, squeezing the bottom bottle forces air up to the top and subsequently water squirts down to the bottom.

Extension – instead of making your own connecting device one called a Fountain Connection can be purchased instead. While not exactly the same type of connecting device as the one presented in this activity, it works in a similar manner. If you do use a Fountain Connection and you find it leaks, put a little electrical tape around the threads of the liter bottles before twisting them together; this does a great job sealing the water inside the bottles.

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Figure 28: the fountain connection squirts water out a straw in the top bottle.

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