Nov. 30, 2005

Sewer Leeches

Observe tiny creatures sink and float, and explore density, life cycles, and the solubility of gases.

Description:

Create imaginary creatures with soda and raisins.

You Will Need:

1. tall glass or clear plastic cylinder
2. clear or translucent carbonated soft drink
3. raisins
4. cloth napkin or towel

Instructions:

Fill the tall cylinder with 7-Up^®, club soda, Mountain Dew^®, or some other clear, carbonated soft drink. Drop a small handful of raisins into the cylinder.

Eventually, small bubbles collect on the raisins. When a sufficient number of bubbles collects on a raisin, it floats to the surface. At the surface, some of the bubbles break and the raisin returns to the bottom.

Occasionally, the bubbles will burst on the raisin's journey upward. Sometimes the raisin will collect bubbles on its way down, before it hits bottom. Also, it will turn and roll as bubbles burst on one side or the other. All of these motions make the raisin appear to be alive, or swimming in the liquid.

Content:

The gas dissolved in carbonated drinks is carbon dioxide, CO₂. Molecules of CO₂ are attracted to tiny sugar crystals on the surface of the raisin. A bubble of the gas is soon formed. When sufficient bubbles collect on the raisin, it becomes buoyant and floats to the surface, unless some of the bubbles break off or burst.

Teacher's Notes

Activity excerpted by permission of the Chemical Educational Foundation (www.chemed.org) from You Be The Chemist. For additional information about these activities and lesson plans, see www.chemed.org/Kit.html.

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