You’ve all seen things float in water. This has to do with an object’s density, or how closely packed its matter is. If an object less dense than water, it will float in it.
But how specifically do things float on the surface of it? This month’s Simple Science Experiment will have you examining exactly where stuff floats and why they float there.
Materials
A tall clear drinking glass, water, Styrofoam packaging
Procedure
Fill the drinking glass halfway with water.
Predict what will happen when you put the Styrofoam piece in the water. It will float, but where?
After you see the results, begin filling the cup the rest of the way up to the top with water (you may want to do this in a sink or on a plate so that you can reduce spilling).
Pour the water in slowly so that you actually fill the water past the top of the lip of the cup. Keep the cup at eye level so you can see what the surface of the water looks like!
What happens to the foam piece now?
Explanation
The first thing to know about this floating issue is that whatever is floating in water will go to the highest point possible in it. So when you have the water halfway filled you’ll notice the foam piece goes right to the side. Every time! Try as you might to get it to stay in the middle of the water, it will always float to the edges. Why? Because the edges are the highest points of the water. But the water is flat you say. Right? Not entirely.
There is a property of water where it sticks to other things called “cohesion.” The water actually sticks to the glass and will pull up around the edges to make a curved surface called a “meniscus.” So when it is half full the foam will always go to the edges because the level of water is slightly higher than in the middle. What about when you fill the cup to the top? If you keep the glass at eye level you should notice a curve to the top of the water. This time it forms a bubble shape with the foam directly in the middle of the cup. Why? Because it is now the highest point, not the edges!
Experiment Further
Do all types of liquids work the same with this experiment? Try out soapy water, maybe juice (don’t waste it though!). What about other things that float? Do they move to the edges as well? Good luck with your experiments!
Steve Davala is a high school chemistry and physics teacher who likes to write and work with Photoshop. He’s got two kids of his own and subjects them to these science activities as guinea pigs. Follow him on Twitteror email him at steve.davala@gmail.com.
