Ultracold + water vapor = crystals (snow!)
This experiment comes from Kenneth G. Libbrecht, a physics professor at California Institute of Technology (Caltech), who studied crystal growth. You can imitate his work by building a snow crystal growth chamber (or diffusion chamber) in your kitchen!
Grab a grown up to help supervise the use of dry ice, hammer, knife, and sewing needle.
WHAT’S GOING ON
Dry ice doesn’t melt, it sublimes. That is, it changes from a solid to a gas when it is warmed, producing carbon dioxide gas in the process.
The bottle becomes a diffusion chamber, in which air is chilled at the bottom and warm at the top. This creates the condition in which crystals can grow.
The water evaporates from the sponge. The water vapor travels around the bottle, until the air inside it gets super-saturated, with humidity at more than 100 percent. Then vapor molecules attach to the string and form a crystal. The string provides a nucleation site where condensation can occur. In the atmosphere, dust crystals perform this purpose! There, supersaturated air condenses into water droplets if the temperature is above 32ºF (0ºC) and into ice crystals (snow) if the air temperature is below 32ºF.