One of my goals with these science videos is to provide support to the homeschool community. After years of writing the Experiment of the Week, and presenting science shows for homeschool groups, I have been impressed with how strongly homeschool families are involved with hands-on science. I have also seen the need for a science curriculum that fulfills their educational needs. Starting with these videos, I hope to offer the homeschool community some of the science education support that they need, as well as a fun way to get students involved with scientific discovery.
I also try to use activities that involve common items. Most homeschoolers don't have access to test tubes, flasks, chemicals, microscopes and other scientific equipment. In the future, I hope to be able to guide them to inexpensive sources for science equipment to enrich their science curriculum.
This is classic science experiment that has been around in one form or another for several hundred years. The name refers to French philosopher Rene Descartes, who does not seem to have had any part in inventing the demonstration. Instead, most sources credit Raffaelo Maggiotte, a student of Galileo's. Even without Descartes, it is a fun way to play while you learn about why things float and sink.
For this experiment, you will need:
- a two liter, plastic soft drink bottle
- a medicine dropper
- water
- a large glass of water
Place the dropper in the glass of water. Squeeze the bulb to let some of the air bubble out. When you release the bulb, water will move in to replace the air you removed. If you release the dropper, it should just barely float in the water. If it floats too high, squeeze it to remove more air. If it sinks, then squeeze out a drop of water and let it take in some air. Then try it again. Keep trying until you reach the point where it just floats.
Fill the two-liter bottle to the top with water. Place the dropper into the bottle and put on the cap. Now, gently squeeze the bottle and the dropper will sink to the bottom. Release the bottle and the dropper rises back to the top.
Understanding the Science
Why does it do this? To understand, we need to know about compressing things and about why things float or sink.
First, lets talk about compression. Generally, gases are easy to compress, while solids and liquids are not. Our soda bottle contains water and air (the air inside the dropper.) When you squeeze the bottle, the water does not get any smaller. Instead, the air inside the dropper is compressed, taking up less space.
Next, we need to understand floating and sinking. If we put an object, say for example, a scoop of ice cream, into a liquid, maybe a nice glass of soda, will it float or sink? All we have to do is compare the density of the object to the density of the liquid. If the object is denser than the liquid, it will sink. If it is less dense, it will float.
OK, then what in the world is density? While it sounds like weight, but there is more to it than that. For density, we compare the weight of a specific sized object with the weight of the same volume of the liquid. So if one cubic foot of ice cream weighs less than one cubic foot of soda, then it is less dense, so ANY sized scoop of ice cream will float in the soda.
On the other hand, if one cubic foot of the ice cream weighs more than one cubic foot of the soda, then it is more dense. That means that any sized chunk of the ice cream would sink.
OK, now lets combine those ideas to understand the Diver. Before you squeeze the bottle, the combination of the dropper and the air weigh less than the same amount of water, and so it floats.
As you squeeze the bottle, the water will not compress. Instead, the air bubble inside the dropper gets squeezed, making it smaller. Now the dropper and bubble still weigh the same, but they take up less space. When the bubble gets small enough, you reach the point where the dropper and bubble weigh more than the same amount of water, and, your Cartesian Diver will sink. When you release the bottle, the air bubble expands back to its original size. Since your Diver takes up more space now, it floats again.
With practice, you can use this for a fun, scientific magic trick. Hold the bottle, and tell the dropper to sink. As you do, gently squeeze the bottle. Then tell it to float, and release the pressure on the bottle. You can make it appear that the dropper is obeying your commands. Then explain the science behind the trick and your audience will be even more impressed.
There are several ways to build on this experiment. As we were taping this video, we noticed that the droppers began to sink without me squeezing. It turned out that a high pressure front was moving through Jacksonville, and the increased air pressure was squeezing the bottle. With several droppers, you could make a barometer for measuring air pressure. Temperature will change things too. As the air in the dropper gets warmer, it will expand. As it gets colder, it will shrink. Once again, this could be used to make a thermometer, telling the temperature by which droppers floated and which ones sank. You can find those in some specialty stores. I even thought for a moment of trying it with ice cream in a full bottle of soda, but that would give results like the famous Mentos experiment, causing a fountain of soda in your kitchen. Not a good idea.
Have a wonder-filled week.
