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.
YOU WILL NEED
- plastic soda bottle, 20-ounce, empty and clean
- knife or scissors
- t pins (from a sewing supply or craft store)
- sewing needle
- thin string (fishing line will also work)
- paper clip
- 2 buckets or other containers. One should be smaller and able to accommodate the soda bottle, leaving an inch or two of space around the sides. The other should be larger and able to accommodate the smaller bucket, leaving an inch or two of space around the sides.
- insulating material such as towels, newspaper, or shipping peanuts
- dry ice—about 5 pounds
- hammer to break up the ice if needed
- 2 plastic bags
- SAFETY EQUIPMENT: heavy insulated mittens or gloves with ice protection coating
STEP ONE: Cut the bottom 1½ inches off the soda bottle carefully. You will use both portions of the bottle.
STEP TWO: Using the bottom of the bottle as a template, cut the sponge so that it fits inside with just a little squishing. Use the pins to fasten the sponge, sticking the pins in from the outside, through the sides of the bottle and into the sponge.
STEP THREE: Thread the sewing needle with the string or fishing line and make a knot at one end. Push the needle from the outside of the bottle bottom through the center of the plastic, through the sponge, and pull through. The string or line should be about 12 inches long. (Hint: Our needle made a hole too big for our knot, and the string kept pulling through the sponge and falling out. So we tied one end of the fishing line to a button, then used the needle to push the end through the sponge again. The button kept the line from falling out.) Tie the paper clip to the end of this line. The paper clip will weigh down the string as it hangs.
STEP FOUR: Place the smaller bucket inside the larger bucket, surrounding its bottom and sides with insulation material.
STEP FIVE: Have an adult help you place dry ice in two plastic bags and carefully break it apart. Pour a layer of dry ice into the smaller bucket. Take the cap off the soda bottle and stand it neck-down in the bucket. Pour dry ice around it until the ice extends halfway up the sides of the bottle.
STEP SIX: Wet the sponge. Place it back in the bottle bottom if you’ve removed it. Fit the bottle bottom, with sponge inside, on top of the bottle that’s in the smaller bucket. Add extra dry ice as needed to keep the temperature cold as the crystals form. Small ice crystals should begin forming after 5 to 10 minutes.
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.
Photographs by Matthew Rakola