Experimenting Cartesian Diver (Grades 5-6)

Focus Question

What causes a Cartesian diver (eyedropper) to sink and float?
Air goes out of the eyedropper. Water goes into the eyedropper.

Generalizations (Key Ideas)

Two objects can't occupy the same space at the same time.
When two object exert a force on each other the object with the greater force moves the object with the smaller force.
Pressure is a force over a particular area.
Objects float if they are less dense than the substance they are in.
Objects sink if they are denser than the substance they are in.
When forces are equal, then there is equilibrium

Concepts

Water takes up space. Air takes up space. Water has volume. Air has volume.
Water is matter. Air is matter.
Objects can exert a force on each other.
A greater force moves an object with less force.
Water flows. Air flows.

My Doing

Experiment Procedure

I will take a 2 liter plastic pop bottle, fill it with water, drop in an eyedropper with some water in it but not enough to make it sink, put the lid on tight, squeeze the bottle to make the dropper sink, and release the bottle to make it float.

I will change the amount of water and air in the dropper and bottle to see how it changes the results.

Results

The water went up into the dropper the more the bottle was squeezed.
It appeared the air left and returned but there were no bubbles were observed leaving the dropper.
The more water in the dropper the lower it floated or sunk.
The more air in the dropper the higher it floated.
The more air in the bottle the harder you had to squeeze to get the dropper to sink.
I couldn't see the rubber on the dropper move - expand or contract.
The more air in the bottle (less water) the more we had to squeeze the bottle to get the dropper to sink, if it did.

See pictures

My Understanding

Transformation of Data Records

Since the air was not observed leaving the dropper it must remain inside. It might be possible that when the bottle is squeezed the air in the dropper is condensed (squeezed into a smaller space/volume) making more room for more water to flow into the dropper. With more water in the dropper system the mass was increased but the volume was the same. This caused the density to decrease and the dropper system (dropper, air and water in it) to sink. If the air wasn't compressed and caused the rubber to expand, then the density of the dropper would not change (the mass and volume would increase). Therefore, I think the air had to be compressed.

Another reason I don't think that the rubber on the dropper expanded was because the force of squeezing the bottle was transferred to the dropper. I mean that the hand squeezed the bottle which squeezed the water, which squeezed the air from two directions. At the bottom through the small opening and at the top by squeezing on the rubber. When this happened the same amount of pressure would be put on both sides of the rubber and it couldn't expand or blow up like a balloon with the air being pushed into it.

Another reason I think the air was compressed is that when we had more air in the pop bottle it was harder to make the dropper sink. When we squeezed the bottle the bottle would really bend in much more before the dropper would sink and when we filled it only half full we couldn't push on it enough to make it sink.

See pictures.

Conclusions

Knowledge Claims

When the bottle is squeezed, the air is pushed into the rubber on the dropper and expands, making more room for water to be pushed into the dropper making it heavier and causing it to sink.

Value Claims

You really need to observe every variable before you can figure out what happens and even then it takes some creative thinking to be able to put the pieces together and you can't be real sure your right even then.

Robert Sweetland's Notes ©