Bubble Suspension
Soap bubbles float on a cushion of carbon dioxide gas
This beautiful experiment illustrates the principles of buoyancy, semipermeability, and interference

(5 minutes or less)

Place a slab of dry ice flat in the bottom of the aquarium. (CAUTION: Use gloves when handling the dry ice; do not touch it with bare skin.) Allow a few minutes for a layer of carbon dioxide gas to accumulate.

(15 minutes or more)

Blow bubbles so they float down into the aquarium. The bubbles will descend and then hover on the denser layer of carbon dioxide gas. After a few minutes, notice that the bubbles begin to expand and sink. Notice the color bands on the bubbles. Notice how some of the bubbles freeze on the dry ice.

As dry ice turns from a solid to a vapor, or sublimes, it produces carbon dioxide gas. Carbon dioxide is denser than air. (Carbon dioxide molecules have an atomic mass of 44 amu [atomic mass units]. Air is made up of nitrogen, 28 amu, and oxygen, 32 amu.) The denser carbon dioxide gas forms a layer on the bottom of the aquarium.

A bubble is full of air. It floats on the carbon dioxide layer just like a helium balloon floating in the air. You might expect that the air in the bubble would cool and contract near the dry ice, but the bubble actually expands slightly. The soapy wall of the bubble allows carbon dioxide to pass through but does not allow air molecules to pass through. Initially, the concentration of carbon dioxide gas is low inside the bubble and high outside the bubble.

The gas gradually diffuses into the bubble, a process called osmosis. The bubble film is a semipermeable membrane--a surface that allows some substances to pass through while preventing others from passing through at all. The cells in your body have the same property. Water, oxygen, and carbon dioxide easily enter some cells, whereas other molecules do not. The added carbon dioxide makes the bubble denser, causing it to gradually sink. The carbon dioxide at the bottom of the tank is cold enough to freeze the bubble.

You can do many experiments with these bubbles.

What happens when bubbles of different sizes collide? Sometimes they make a single larger bubble, other times they join as two bubbles with a flat or bulging wall between them. If the two bubbles are the same size, the wall is flat between them, since the pressure is equal on both sides. If the two bubbles are of different sizes, the wall will bulge away from the smaller of two bubbles, since the smaller bubble will have a higher pressure inside.

How does a bubble respond to a comb that has been charged by rubbing it with a wool cloth? The neutral bubble is electrically polarized by, and attracted to, the charged comb.