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Convection Detection

Science Snack
Convection Detection
Create a convection cell inside a CD case.
Convection Detection
Create a convection cell inside a CD case.

A bag of colored water inside a CD case shows the mesmerizing movement of a fluid undergoing convection. 

Tools and Materials
  • Safety goggles
  • CD jewel case
  • Pliers
  • Scissors
  • Two paper or plastic hot-drink cups that are easy to cut with scissors
  • Plastic sandwich bag (1 quart size)
  • Room-temperature water
  • Two strong rubber bands
  • Warm water (hot tap water)
  • Cold water
  • Two disposable pipettes
  • Red food coloring
  • Blue food coloring
  • Optional: tray (helpful for containing spills)
Assembly
  1. Put on your safety goggles.
  2. Open the CD case and stand it up horizontally, the way you might open and stand a book on a desk. Then, using pliers, pinch down over the top lip of the case and break a hole in the case’s “front cover,” about an inch down from the top (or however far down your pliers will go), and an inch over from the far left side (see photos below). The hole just needs to be large enough to accommodate your pipette.
  3. Using the same process, pinch a second hole on the same side (that is, still on the case’s front cover), an equal distance to the left of the hinge (see photo below). When you’re done, you’ll have two holes in the top of the case’s left side—one near the far edge, and one near the hinge. There should be no holes on the “back cover.”
  4. Using scissors, cut a slit down through the side of one of the hot-drink cups. It should be just wide enough to accommodate the width of the closed CD case. Cut about halfway down the cup.
  5. Repeat Step 4, cutting a slit into the second hot-drink cup. Be sure the cuts to both cups are even (see photo below). You’ll be using them as holders—one for each side of your CD assembly.
  6. Put about two finger-widths (3-4 cm) of room-temperature water into the plastic bag (see photo below).
  7. Place the bag inside the CD case, with the open end at the top of the case, where you made the two holes. Make sure the bottom of the bag is touching the bottom of the case (see left photo below). Gently close the case and use rubber bands to secure the water-filled bag in place (see right photo below). Try to make the plastic bag as wrinkle free as possible.
  8. Place the CD case into the slits you cut into the hot-drink cups—one on each side, so they act as a stand to keep the case level (see photo below). (If you’re using a tray, put the cups on the tray first, and then place the CD case into the slits in the cups.)
To Do and Notice

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.

What's Going On?

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.

Teaching Tips

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.