From Anne Marie Helmenstine, Ph.D.,
Your Guide to Chemistry.
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Dry ice is the solid form of carbon dioxide. You can use dry ice to freeze bubbles solid so that you can pick them up and examine them closely. You can use this project to demonstrate several scientific principles, such as density, interference, semipermeability, and diffusion.

Materials

• Bubble Solution (from the store or make your own)
• Dry Ice
• Gloves (for handling the dry ice)
• Aquarium or Box

Procedure

1. Using gloves to protect your hands, place a chunk of dry ice in the bottom of an aquarium or box (aquarium is nice because its sides are clear).
2. Allow about 5 minutes for carbon dioxide gas to accumulate in the container.
3. Blow bubbles down into the container. The bubbles will fall until they reach the layer of carbon dioxide. They will hover at the interface between air and carbon dioxide. The bubbles will start to sink as the bubbles cool and the carbon dioxide replaces some of the air within them. Bubbles that come into contact with the dry ice chunk or fall into the cold layer at the bottom of the container will freeze! You can pick them up for closer examination (no gloves needed). The bubbles will thaw and eventually pop as they warm.
4. As the bubbles age, their color bands will change and they will become more transparent. The bubble liquid is light, but it is still affected by gravity and is pulled to the bottom of a bubble. Eventually, the film at the top of a bubble becomes so thin that it will open and the bubble will pop.

Explanation

Carbon dioxide (CO₂) is heavier than most of the other gases present in air (normal air is mostly nitrogen, N₂, and oxygen, O₂), so most of the carbon dioxide will settle to the bottom of the aquarium. Bubbles filled with air will float on top of the heavier carbon dioxide. Here's a tutorial for calculating molecular mass, just in case you want to prove this for yourself!

Notes

Adult supervision is recommended for this project. Dry ice is cold enough to give frostbite, so you need to wear protective gloves when handling it. Also, be aware that extra carbon dioxide is added to the air as dry ice vaporizes. Carbon dioxide is naturally present in air, but under some circumstances, the extra amount can present a health hazard.

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