Carbon dioxide (CO2) is heavier than most other molecules and elements in the air.  In the same way that Helium rises (think of party balloons), CO2 sinks.

With a very large bowl or container and lots and lots of vinegar and baking soda, you can make a little “puddle” of CO2.  When soap bubbles - which are filled with normal air - hover over this puddle, they float in place like beach balls sitting on top of a pool.  You can see this in the picture, but of course it’s much weirder-looking in person.  Bubbles serenely unmoving in midair.

A Giant Bowl. You need a bowl big enough that the CO2 can slosh around without completely seeping away.

Recipe for CO2. A back-of-the-napkin calculation shows that one big box of Baking Soda is good for about six big bottles of vinegar.  Their reaction produces CO2 gas and a big mess.

Bubbles! The classic recipe is 12 parts water to 1 part blue Dawn dish detergent.  A few tablespoons of glycerin (which can be bought at a pharmacy) help the bubbles last longer.  We tried some variations, like all-glycerin bubbles.  It also seemed like a good occasion for the Bubble Thing!

Finding the CO2 line. This Four Gas Tester, designed to check if air in a workplace is safe to breathe, showed that the air was normal above the bowl.  But when I dipped the sensor under where the bubbles were floating, red alarm lights indicated that the air was not breathable - too much CO2!

1. Get a medium-sized plastic soda bottle and a party balloon.

2. Puncture a hole in the side of the bottle near the bottom.  It’s easy if you use a nail heated with a match or a candle.

3. Dangle the balloon inside the bottle and open it over the bottle’s lip.  (See photo.)

4. Inflate the balloon inside the bottle.

5. With your mouth still on the bottle, put a finger snugly over the hole in the side.

6. The balloon stays open on it’s own!  Fill it with water from a faucet.

7. Bring it outside, and take your finger off of the hole…

Science! To quote Zap Science (see section “Soda Bottle Science”), we live at the bottom of an ocean of air.  Air pushes in upon everything.  When the hole in the bottle is uncovered, air pushes on the balloon from all directions.  When the hole is covered, there is air pushing in on the balloon but, thanks to your finger, there is not much air pushing back.  The pressure of the atmosphere - all the air piled up over your head - is working to hold the balloon open.  When you uncover the hole, you let the atmosphere back in (so to speak) and - splash! - the balloon is free to shrink again.

This powder, “super-absorbent polymer” is used in disposable diapers to help prevent leaking.  Each little grain of powder swells into a gelatinous bead that traps water.  (See the last photo.)

Where can I find this stuff? The easiest way to get it is to tear apart an (unused!) diaper or two over a cookie tray.  Really rip apart the cottony insides of the diaper, releasing the white flecks of powder that are dispersed through it.  Tip the tray to shuffle the powder into one corner, and collect it.  Discard the rest of the diaper.

The Second Act. You’ve seen the first part: simply stir a spoonful of powder into a glass of water.  The water will be “solid” within a minute or two.  But there’s more!  Add some sidewalk salt (active ingredient: calcium chloride) to the glass and mix it in.  The salt effectively disables the polymer, and water comes back out.  Try it with table salt too (sodium chloride). What happens?

Safety Note. The super-absorbent polymer is safe - it’s in diapers, remember - but if you’re collecting lots if it, avoid breathing its dust.  Sidewalk salt, while not too serious, is less people-friendly.  Safety glasses and dishwashing gloves are recommended.  Wash the glass thoroughly after this demo.  If you do the part with the sidewalk salt, use a plastic cup that you can throw away.

Glowing Glass. These glass squares look plain and unremarkable. Under Ultra Violet light, they become brilliantly colored: red, orange, yellow, green, blue, and purple. Each square is doped with a different chemical that fluoresces under UV light, producing a vibrant rainbow.  They were made by an expert in this sort of chemical trickery, Dr. Matthew Dejneka.

The flashlight lighting the glass is also pretty cool technology: it has four UV LEDs. To our eyes, it looks dim and vaguely purple, but it is extremely bright in the (invisible) UV range.

Weird Magnification. A usual magnifying glass bends the light reflecting from an object to create a larger image. This “taper optic” works in a totally different way. Notice how it is different from a magnifying glass:

• A magnifying glass distorts the image, especially around the edges. The taper optic’s image is flat.
• To read a magnifying glass, you have to move it around to get the words into focus. The taper optic sits flat on the page, and the words look as if they are written in large print directly on the glass.
• A magnifying glass focuses a bright spot at the center of its view and it darkens the rest. The taper optic brightens everything evenly.

What is it? It is bundle of tiny glass fibers, fused together into a solid chunk of glass. The fibers are thin toward the bottom and thicker toward the top. Each fiber carries a piece of the image and spreads it out on the top surface, enlarging the print. (It’s a two-way street: Fibers also carry light down, concentrating all the light that falls onto the top surface into a small area on the page, making the page a little brighter.)

Why haven’t these replaced magnifying glasses? They are too expensive. A good-sized one is in the $80 range, and as of this writing they are not manufactured by anyone. I got mine by emailing the inventor, Harvard Professor Eli Peli. Mine is slightly marred, but it came free for educational use!

What happens if you flip it up side down? This.

Sharing the science at the Corning Museum of Glass.

Also, I recommend books by Martin Gardner, particularly Entertaining Science Experiments with Everyday Objects.  The books are a little dated, but they have ideas you won’t find anywhere else.

They are secretive about their more elaborate geysers.  Here are my ideas.

Magnetic Trigger. Grobe and Voltz suggest hanging the mentos inside the bottle on a short string held by a paper clamp.  For a quicker and more elegant trigger, use small strong magnets.  (One goes on the string inside the cap; one sits on top of the cap until it is removed, letting the Mentos drop into the Diet Coke.)  With a couple cheap electromagnets - even ones homemade from a nail coiled in wire - you could trigger multiple eruptions in perfect unison at the flip of a switch.

Whirling Bottle. In immitation of one of their geysers, I drilled holes in the side of the bottle above the liquid.  I suspended in the bottle from a wound string, and let it spin.  I want the eruptions to come from the sides, not the top, so I used the magnetic trigger.  (That way, you don’t need a hole in the cap.)

Science! Grobe and Voltz link to two good explanations.  The recipies for Mentos and Diet Coke are, of course, secret.  Exactly what happens is an unsettled question.  It seems to be a physical interaction having to do with the surface of mentos and the carbonation in Diet Coke, not a chemical reaction.

Other Results?

xkcd.com. Used with permission, under the Creative Commons Licence.

Get four popsicle sticks, preferrably after eating four popsicles.  (Use normal thin ones, not thick ones from a craft store.)  Assemble them as shown in the photos.  Weaving in the last piece is just a little tricky.  You’ll have to bend the sticks a bit.

Notice, in the final picture, that the sticks at the top are barely overlapping.  This is your “popsicle stick bomb.”  Throw it!

Made in a large supply, these are great for rubber-band-gun target practice.  Made carefully, they can be “chained” together.  …This is how I spent my childhood.

Science! If you want to slap an “educational” label on this most excellent of pasttimes, consider it a lesson in potential energy (bending the sticks) converting into kinetic energy (boom!).

Advanced Popsicle Stickmanship. A leaner shape makes a more powerful popsicle stick bomb. That is, squeezing the whole shape in from side to side bends the sticks farther and makes the “explosion” better.

1. Water + Baking Powder.  Not as foamy, but a little safer for small kids (no eye-stinging vinegar).

2. Vinegar + Baking Soda + Dish Soap.  The soap traps all the CO2 gas, and you get much more foam.

For more ideas in the spirit of Vinegar + Baking Soda, see Diet Coke + Mentos Variations.  For a more refined Vinegar + Baking Soda science demo, see Hovering Bubbles.

If you remember the words well enough to sing them, the reliability is even better, but it does really well even if you can only hum ten seconds of the tune!  The service, dubbed Midomi, was launched in January 2007 - not sure why this isn’t better known.

(Thanks for telling me about this, Shawn!)