Bouncing Raisins

Age

Elementary School

Format

Hands-on

Materials

   Raisins
   Roughly 40 mL of vinegar per performance
   A heaping teaspoon of sodium bicarbonate (baking soda or alka seltzer) per performance
   A skinny glass or graduated cylinder 

Safety Precautions

Science Theatre demonstrators must keep the safety of themselves and their audience in mind at all times. All Science Theatre demonstrators must have read through the Safety Training page. The ST Safety Box with first aid kit, fire extinguisher, etc. should always be available to demonstrators. Always wear safety gloves, glasses, and a labcoat if handling chemicals; always perform potentially dangerous demonstrations at a safe distance from the audience; and always keep a very close eye on any volunteers you call from the audience. Whenever a demonstration involves food, students may ask to eat or taste the materials - NEVER let the students eat demonstration materials. You may explain that the materials have been near other, more harmful chemicals that we use in other demonstrations.

   Vinegar/Acetic Acid MSDS
   Sodium Bicarbonate MSDS 

Preparation

Assemble your materials.

Demonstration

Fill the glass with the vinegar and SLOWLY add the baking soda. If you do it too fast, you will get an acid/base volcano! Let the foam settle back down. Add raisins to the mixture. You will see CO2 bubbles collect on the raisin. Once enough bubbles collect, the raisin will float. If the raising hits the surface of the liquid and the bubbles leave the raisin, the raisin will fall back down. The bouncing may take a few minutes to get going. You can watch the raisins bounce for at least 20 minutes or more! However, if you are doing the demonstration for several groups of students, you should make a new solution each time so that they can see the chemicals you are using.

Note for those using alka seltzer instead of plain baking soda: the alka seltzer seems to make the solution much more fizzy than baking soda and, as a result, the raisin can be harder to see.

What to Say

Here I have some baking soda and vinegar - these are two chemicals that your parents probably have in the kitchen. They're used in cooking and baking, and also cleaning.

Have you guys ever made a volcano out of baking soda and vinegar? It's really easy to do - you just need to mix vinegar, which is an acid, and baking soda, which is a base, together. You get an acid/base reaction which produces a ton of carbon dioxide bubbles! It's pretty messy, so don't do it without help from your parents.

Carbon dioxide is just a gas - it's part of the air we breathe. It's also the stuff in soda that makes it so fizzy!

When I mix the baking soda and vinegar, you will see lots and lots of CO2 bubbles forming!

OK, now I'm going to add some more food to the mixture. These are just raisins from the grocery store.

What's happening to the raisins? They're bouncing! Can anyone guess why?

Let me give you a hint - things that are less dense than water float in water. To be less dense just means that a bit of it weighs less than the same amount of water. Our mixture of baking soda and vinegar is really mostly water because the vinegar is very watered down.

Well, those carbon dioxide bubbles are gas. Like almost all gases we see around us, carbon dioxide is much less dense than water. Do you see the gas bubbles collecting on the raisin? The bubbles definitely float in water, so if you cover the raisin in lots of bubbles, they will help the raisin float.

Do you know why the raisin sinks back down? It's because it's losing some of those bubbles! If the raisin has less bubbles on it, its overall density is higher. If its density is higher than water, it will sink.

Why It Is

Vinegar and baking soda react together as acids and bases, respectively, and produce carbon dioxide. More specifically, the baking soda dissolves in the water to produce bicarbonate ions (HCO3):

NaHCO3 ---> Na+(aq) + HCO3-(aq)

At the same time, the vinegar dissociates and produces hydrogen ions:

CH3COOH <--> H+(aq) + CH3COO-(aq)

These two products form carbonic acid:

H+ + HCO3- ---> H2CO3

Finally, the carbonic acid decomposes into carbon dioxide gas:

H2CO3 ---> H2O + CO2

The carbon dioxide gas produces gas bubbles in the water that collect on the raisin and cause it to float. The CO2/raisin pair floats because it is of much lower density than water. The density of water is 1 g/ml, but the density of CO2 is only 0.00197 g/ml! Even a little bit of CO2 can lower the average density of the CO2/raisin pair by a lot. Once the density of the pair is a little lower than water, it floats! When some CO2 is lost and the density edges closer to that of the raisin itself, which is higher than water, it sinks.

Real Life Examples

Carbon dioxide dissolved in soda produces soda's fizzy sensation on your tongue.

A helium balloon floats because the helium gas is less dense than air.

Also see the Exploding Soda and Density Rainbow demonstrations.