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A bag of colored water inside a CD case shows the mesmerizing movement of a fluid undergoing convection.
Add hot water to one of the hot-drink cups, up to the level of the bottom of the CD case. Then add cold water to the other cup, also up to the level of the bottom of the CD case. Make sure the bottom of the CD case is touching the water in each of the cups.
Fill one pipette with a few drops of red food coloring. Working on the hot-water side of your setup, carefully insert the pipette into the bag and through the hole in the CD case. Squirt a small amount of red food coloring at the bottom of the plastic bag. Try not to squirt in any air bubbles. Then carefully remove the pipette from the CD case.
Repeat this on the cold-water side of your setup, this time using blue food coloring. Then seal the bag tightly.
Now the show begins: What do you notice? Keep observing the ongoing changes within the CD case.
The gorgeous swirls of color you see are created by convection, a highly efficient form of heat transfer that drives the circulation in the earth’s atmosphere, oceans, and mantle.
Convection can only take place in fluids—that is, liquids and gases—and it relies on changes in density. Heating causes the water to expand in volume, making it less dense. Cooling, meanwhile, causes the water to shrink in volume and become more dense.
These density changes put fluids into motion: Inside your CD case, you can see the cooler, denser, blue water creep across the bottom toward the warmer side, while the warmer, less dense, red water is buoyed upward.
In time, the blue water on the warm side of the case gets heated too, and gets pushed upward by the continuing approach of the cooler water from the cold-water side. Meanwhile, the red water moves across the top of the container toward the cold side, losing heat as it travels, eventually cooling enough to sink.
The cycle of heating and cooling continues, creating a convection cell, a circular pattern of rising and falling fluid.
Ask your students to predict what will happen before the food coloring is added. Predictions are a great way to get students to think, and will get them even more interested in seeing what happens. Better yet, provide colored pens or pencils so that students can draw their predictions.
This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.
Attribution: Exploratorium Teacher Institute