Dancing Raisins
Most of you have seen the basic description of this demo/LAB. Place a few raisins in a beaker or glass of Seltzer Water or 7-UP or some such beverage. The raisins will sink. However, upon watching, they will begin to form little bubbles on them and, sure enough, rise to the top where they will again sink and continue this eerie round trip. Well, the only problem is, it usually doesn't work. There are some factors involved here that will increase your chances of success.
- Soak the raisins in water for a few hours before the Demo. Overnight is cool! This way, if they are dried out at all, they needn't spend time absorbing the Seltzer just to become buoyant. This provides them with a density almost the same as water and they bob up and down easily.
- Use FRESH Seltzer, 7-UP, Tonic Water. The BEST thing I've ever used is some cheap Champagne! It bubbles vigorously! And clean-up with Champagne is more fun than 7-UP...
- Cut the raisins in half. This way, less mass to pick up. Works quicker.
Linda Smith, PAEMST 2000, uses a tricky little thing with these guys. She has 3 identical jars with lids. One is dark opaque liquid. One is dark-ish, but translucent. The other is clear. In the 2nd and 3rd, you can see these raisins bobbing up and down. She introduces this as a "Biological method of cleaning NJ's waterways". She tells (lies to) the audience that these are 'bugs' that consume oil in the water and turns it into regular dissolvable stuff that can be filtered out by regular means. The audience is usually quite impressed till she tells them that the jars #1 & #2 just contain different amounts of dark Karo syrup and the 3rd is clear water and the 'bugs' are raisins! Cool!
SCIENCE BEHIND THIS? The raisins offer nucleation sites for bubbles, usually CO2, to form. As the bubbles collect, the raisin feels an upward force like a small kid holding 1 gazillion He balloons. Upon rising, the bubbles swell and burst due to the decreased pressure. The raisins, now with decreased 'lift', fall back down into the liquid to begin this all over again.