Rising Raisins
For once it is okay to play with your food.

Concepts
Density, saturation, nucleation, data analysis

Materials
  • Clear cup or 250 mL beaker
  • Clear carbonated soda (such as Sprite, Sierra Mist, seltzer water)
  • Fresh box of raisins

Procedure
  • Pour soda into the beaker or cup until it is ¾ of the way full
  • Drop 4 or 5 raisins into the soda.
  • Observe carefully for two minutes
  • Clean up your mess

Analysis Questions
1. Initially, how does the density of the raisins compare to the density of soda? Explain your reasoning.

2. Over time, what must happen to the density of the raisins to cause them to float?

3. What causes the density of the raisins to change over time?

4. Assume you observed one of the raisins rising and falling in the soda with a period of about a minute. Which graph below best depicts the change in density of the raisins
over time? Explain your reasoning.




Explanation
In general, raisins are denser than water and will sink to the bottom of cup of water. However, if one adds a little carbonation to the water, the carbon dioxide gas will collect on the raisins and the combined density of the raisin-carbon dioxide bubbles will be less than the water, causing it to rise. Once the raisin-carbon dioxide bubbles reaches the surface of the water the bubbles escape and the lone raisin once again falls to the bottom of the cup. The process is then repeated as bubbles once again accumulate on the raisin.

Soda is a liquid supersaturated with carbon dioxide gas, and nucleation sites are places where the carbon dioxide can make bubbles. A nucleation site can be a scratch on a surface, a speck of dust, or any place where there is a high surface area to volume. Raisins have a plethora of nucleation sites; lots of twists and turns and crevices that allow bubbles to form. Carbon dioxide can quickly accumulate in several different places on the raisin simultaneously causing it to rise up after a brief amount of time in the soda.

figure35.jpg
Figure 35: Raisins in the water do not rise (left), while raisins in soda do rise (right).

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