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Home : 2003 : Jan : 31

    Ok, I'll give it a try
    By Julianne

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    I actually had to look up convection to be sure I had the right concept! I work with younger children and use this experiment as a teacher demo to help explain density. The kids love it, and there's no reason why older students shouldn't do it themselves. You need a large glass container for each student or group. A mayo jar or canning jar will work. Then you need a much smaller jar with a narrow mouth. If you can get those tiny tabasco bottles, they work. So do the small, glass bottles with screw-on lids that many science stores sell. Look around. I've found usable bottles lots of places.

    Now, tie a string around the neck of the small bottle. Fill the large bottle with cold water. Place one drop of food color in the small bottle, then fill it up with hot hot water. Have the students predict what they think will happen when they lower the small bottle into the larger one. Then have them carefully dangle the small bottle down into the larger one until it comes to rest on the bottom of the jar. The colored liquid should stream out of the small bottle and head straight up. When it hits the surface of the water it will cool and start to spread out, some of it will begin to mix with the non-colored water, but some may actually get cold enough to sink down to the bottom of the jar.

    Now you can repeat the experiment by reversing the hot/cold combination. Fill the large jar with hot water, the tiny one with food color and COLD water. What will happen this time? It's fun to leave the jars in place for several days and see if there are further changes.

    As a follow-up you can have the students draw the experiment and explain what happened and why.

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