Expt 018 -- CO₂ Crystal Ball

Have you ever gazed into a cloudy crystal ball...made from a soap film? Dry ice is added to a bucket of water. A soapy cloth is drawn across the lip of the bucket to create a soap film sheet. This film gradually inflates into a misty, colorful crystal ball that undulates gracefully to the air currents in the room! Gentle puffs of air set up resonance patterns in the crystal ball. This dramatic demonstration is a must around Halloween time.

Chemical Concepts

1. Dry ice is the solid form of carbon dioxide. At normal atmospheric pressure, it sublimes (changes directly from a solid to a gas) rather than melts.
2. Different gases can have very different solubilities in a given solvent. For instance, CO₂ is considerably more soluble in water than is N₂.
3. Light reflects off of the outside as well as the inside surface of a soap film. The water in the film tends to gravitate toward the bottom, making the lower portions of the film become progressively thicker. These two phenomena cause interference patterns in the reflected light, resulting in horizontal bands of spectral colors.

Safety

• Use gloves for thermal protection from the dry ice.
• Wear eye protection while handling dry ice.

Procedure

1. Roll the cloth into a strap, 3-4 cm in width and 10-20 cm longer than the diameter of the bucket. Soak the strap in the soap solution, then take it out and let most of the excess solution drip off.
2. Fill the bucket about half-way with tap water. Drop in the dry ice and observe as the fog eventually spills over the rim.
3. Draw the soapy strap slowly over the rim of the bucket to create a soap film "lid." This may take several attempts; it helps to have the rim somewhat wet to begin with. The humidity increases as you work. Avoid dripping the soap solution into the bucket.

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4. Point out the interference patterns of the reflecting light.

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5. As the film slowly inflates into a dome, blow gentle puffs of air at the dome and observe the way it resonates. Puff gently at regular intervals to set up waves. Change the interval to change the shapes observed.

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6. The wave patterns are more interesting from the top.

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Variation:

After the dry ice has been added, place a lit lantern-type flashlight face-up in the bucket, then draw the soap film across. The crystal ball now glows in the dark! Colored filters may be placed over the light to produce different colors! Or, for an even more mysterious demonstration, cut out a large, clear plastic circle (from an overhead transparency sheet), color it like a pie chart with permanent markers or colored "gels" from a theater supply store -- one-third green; one-third blue (cyan); one-third red (magenta), and use Velcro to attach the circle to the crank of some wind-up toy or music box strapped to the side of the flashlight. As the crank slowly unwinds, the crystal ball gradually changes from one color to the next! This variation is mostly just a visually appealing display, although it does demonstrate additive color mixing through the scattering of light.

Questions

1. Why does the dome stop growing even though the dry ice is still subliming?
2. How does the wave action change as the time between puffs decreases?
3. How does CO₂'s density compare to the density of air? What observation illustrates this density difference?

Handout Makeup

Name ___________________________ Class ________

Teacher __________________________

BeckerDemos 018 CO₂ Crystal Ball

Watch the movies. Use the movies and pictures to answer the questions.

Curriculum-

• This activity is a must around Halloween time. Like the shrinking suds activity, it can be incorporated into a discussion on osmosis or diffusion across membranes, gas solubility, or a unit focusing on the properties of carbon dioxide. This variation is intended more for entertainment than for any extended understanding of the principles involved.
• Alternatively, use the demonstration to help students visualize waves and resonance patterns. The horizontal rainbows of color circling the dome are due to the interference patterns of the reflecting light. Blow gentle puffs of air at the dome and observe the way it resonates. This effect could be used in conjunction with the time-honored vibrating string analogy for modeling electron energy levels as harmonic frequencies.

Activity-

Demonstration - Student or Teacher

• You probably want to do this dramatic demonstration yourself, but the demonstration is safe enough for student projects.
• The CO₂ Crystal Ball is part III in a Dry Ice/Soap Film Quartet of Activities.

Time-

Teacher Preparation: 3-5 minutes

Class Time: 5-10 minutes

Materials-

• 500-700 g of dry ice (one or two fist sized chunks)
• 200 mL of 5% solution of Dawn or Joy (200 mL -- In a bowl, mix approximately 10 mL detergent with 200 mL of water. Let sit.)
• a large bucket or can with a smooth flat rim
• a bowl or wide-mouth cup
• an absorbent cloth (a piece of old t-shirt preferred) or paper towel
• thermal gloves for handling dry ice

Optional:

• a lantern-type flashlight

Disposal-

Allow dry ice to sublime.

Lab Hints-

• Forming the film may take several attempts; it helps to have the rim somewhat wet to begin with. High humidity also helps. The humidity in the area increases as you work.
• If the air is too dry, it may be impossible to form a film. Use the Shrinking Suds (016) or Shrinking Bubble (017) demonstration instead.
• Also, try to avoid dripping soapy water into the bucket, for it will merely lead to a repeat of the "Shrinking Suds" demonstration.

Observations-

• Dry ice is frozen carbon dioxide. Rather than melting, though, dry ice sublimes -- that is, it changes directly from the solid state to the gaseous state. Thus the gas produced in the bucket is CO₂. But carbon dioxide, like most gases, is invisible. The fog that you see is actually H₂O in the liquid state: small, suspended droplets of condensation produced as warm vapor from the water comes in contact with the cold subliming CO₂-- like the fog you see when your warm, humid breath is exhaled into cold winter air.
• If left undisturbed, the film quickly produces intense horizontal rainbows of color circling the dome. These are due to the interference patterns of the reflecting light. This happens in all soap films as a result of varying film thicknesses, but these colors seem especially pronounced in the crystal ball, perhaps because of the misty white backdrop.
• Once the soap film lid is established, it quickly forms into a misty dome -- resembling quite dramatically a fortune-teller's "crystal ball," only considerably more fluid and pliable. Strategically blown puffs of air can set up any number of resonance frequencies. (This effect could perhaps be used in conjunction with the time-honored vibrating string analogy for modeling electron energy levels as harmonic frequencies!)
• When you blow regular puffs of air across the film. The bubble vibrates with pronounced nodes. The puffs are adding energy to the system. Most systems vibrate with a natural frequency to dissipate the energy. The behavior is similar to that observed with a vibrating string. Changing the rate of puffing changes the number of nodes and the shapes observed.
• If left long enough, another peculiar observation can be made: the dome stops growing, even though one can still hear the dry ice subliming vigorously in the bucket below. The explanation for this phenomenon centers around the relatively high solubility of CO₂ and its ability to diffuse readily through the soap film. First let us assume that the CO₂ is subliming at a more or less constant rate, and that this rate of sublimation is considerably greater than the rate at which the CO₂ can diffuse through the original soap film "lid" drawn across the bucket. The film is thus pushed outwards into a dome. As the film grows, however, its surface area increases and its thickness decreases. Consequently, the rate at which the CO₂ can diffuse through it increases. Hence, an equilibrium-like state is eventually reached where the rate of diffusion is equal to the rate of sublimation. In short, the dome reaches a point where it is "leaking" as fast as it is being filled, and so its size remains more or less constant.
• Actually, of course, the sublimation rate is decreasing as the dry ice is used up. At first, this deceleration is too gradual to play much of a role in the demonstration, but as the chunks of dry ice begin to dwindle (and as the water temperature drops and layers of ice begin to form on the dry ice, insulating it from the rest of the water) the rate of sublimation can decrease quite substantially. When this happens, if the soap film has lasted long enough, one can see the dome start to shrink back down into the bucket.

Answers-

Q1. Why does the dome stop growing even though the dry ice is still subliming?

A1. The carbon dioxide gas is diffusing through the soapy water membrane.

Q2. How does the wave action change as the time between puffs decreases?

A2. The waves change frequency and shape. More nodes form at higher frequencies.

Q3. How does CO₂'s density compare to the density of air? What observation illustrates this density difference?

A3. CO₂ is much more dense than air, and cold CO₂ is even more dense still. This greater density is illustrated by the quick spilling over of the mist-filled CO₂ gas whenever the large bubble pops.

Reference-

• Soap-bubbles, Their Colours and the Forces Which Mold Them. C. V. Boys. Dover Publications, Inc., 1959.
• "Problem Solving with Soap Films: Part I." C. Isenberg in Physics Education, Vol. 10, pages 452-456; September, 1975
• "Problem Solving with Soap Films: Part II." C. Isenberg in Physics Education, Vol. 10, pages 500-503; November, 1975.
• Tom Noddy's Bubble Magic. Tom Noddy. Running Press, 1988.

Key Words 1-

waves, resonance, solubility, gases, diffusion, interference, solubility, sublimation, dry ice