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Yesterday I preformed a fun little experiment where I poured a glass of seltzer water into a glass, then dropped a few raisins into it.

I observed that the raisins would float at the top of the glass for a few seconds, then sink all the way to the bottom, then float back up, and repeat seemingly without end.

Why does this happen?

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The raisin has nucleation sites on it that allow bubbles of $\ce{CO2}$ to form. The raisin is light enough to be lifted by the bubbles as they push their way to the surface. As the bubbles are released into the atmosphere, the raisin once again sinks until more bubbles form on it. This will continue until the soda water has lost the majority of its dissolved $\ce{CO2}$.

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    $\begingroup$ The rest of the story is that a raisin (without CO2 bubbles) is denser than water and would sink in uncarbonated water. Also for this experiment the difference in density between carbonated water and uncarbonated water is negligible compared to the difference in density between water and a raisin. $\endgroup$ – MaxW Sep 2 '16 at 5:18

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