Expt 017 -- Shrinking Bubble

Dry ice is added to a flask half-filled with water, and a fog rises to the rim. A soap film is drawn across the top of the flask. This film gradually inflates, stops growing, and finally shrinks as the CO₂ is depleted. CO₂ is clearly diffusing through the soap film as the bubble stops increasing in size while CO₂ is still bubbling out of the water. Rings may be visible on the bubble.

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. Diffusion of a gas through a liquid depends on solubility of that gas in the liquid.

Safety

• Use thermal gloves to handle the dry ice.
• Wear goggles. Dry ice can damage eyes.

Procedure

1. Fill the flask about half-way with tap water. Use a single piece of dry ice that is about the size of a grape (2 mL). Drop in the dry ice and observe as the fog eventually spills over the rim. At this point the air has been pretty much flushed out.
2. Dip your finger into the detergent solution. To create a detergent film "lid", draw the solution over the rim of the flask with your finger. This may take several attempts. Avoid dripping the detergent solution into the flask.

   !!!Click here to See Movie.

3. Observe the size of the bubble. Note the volume of gas bubbling in the flask and the size of the bubble.

   !!!Click here to See Movie. The movie is accelerated 5x actual speed.

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

   !!!Click here to See Movie. The movie is accelerated 3x actual speed to enhance the movement of the rings.

Questions

1. Why does the bubble stop growing even though CO₂ gas is still forming in the flask?
2. Calculate the volume of gas produced if 3.0 g of CO₂ sublimes. Assume the gas is at STP.
3. If the flask holds 200 mL of gas above the liquid, explain what happens to the extra gas.

Handout Makeup

Name ___________________________ Class _______

Teacher __________________________

BeckerDemos 017 Shrinking Bubble

Watch the movies.

Use the movies to answer the questions.

Curriculum-

Use this experiment when discussing gas solubilities and diffusion. It is useful as an introduction to the idea of equilibrium processes. It is much easier for students to see the diffusion processes in this experiment than in either the crystal ball or the shrinking suds. Both the subliming CO₂ bubbles and the bubble size are visible.

Activity-

Demonstration - Student or Teacher

• This demonstration works well for a classroom or a larger group.
• The Shrinking Bubble is Part II in a Dry Ice/Soap Film Quartet of Activities.

Safety-

• Use thermal gloves to handle the dry ice.
• Wear goggles. Dry ice can damage eyes.

Time-

Teacher Preparation: 5 minutes

Class Time: 5 minutes

Materials-

• 20 g of dry ice
• 100 mL of 5% solution of dish detergent (100 mL -- In a bowl, mix approximately 5 mL detergent with 100 mL of water. Let sit.) (Joy^® or Dawn^® work best)
• 1 large Erlenmeyer Flask (1-liter works well)
• thermal gloves for handling dry ice

Disposal-

Allow the dry ice to sublime.

Lab Hints-

• The bubble film lasts longer on humid days. On dry days in the middle of winter, the bubble may burst before it shrinks.
• The appearance of rings is most pronounced on thin bubbles just before bursting. If the bubble bursts prematurely, wait a moment for CO₂ evolution to slow down and apply a thicker layer of detergent solution.
• Use one small chunk of dry ice. Start with one piece about the size of a grape (2 mL). If evolution of gas is too fast, wait a few minutes for the size to decrease and try making the film again. If evolution of gas is too slow, add another small chunk of dry ice.
• Do not add powdered dry ice; it gives a sudden burst of gas instead of a steady stream.

Observations-

• If the film lasts long enough, a peculiar observation can be made: the dome stops growing, even though one can still hear and see the dry ice subliming vigorously in the flask 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 flask. 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.
• Discuss the reasons for the constant bubble size even when subliming is still clearly visible. This can serve as a good introduction to equilibrium.
• Actually, of course, the sublimation rate decreases as the dry ice is used up. Initially, this deceleration is too gradual to play much of a role in the demonstration. As the dry ice is used up the rate of production of CO₂ diminishes significantly and finally stops as the dry ice is consumed. As this happens, if the soap film has lasted long enough, one can see the dome start to shrink back down into the flask.

Answers-

Q1. Why does the bubble stop growing even though CO₂ gas is still forming in the flask?

A1. 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 flask. 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.

Q2. Calculate the volume of gas produced if 3.0 g of CO₂ sublime. Assume the gas is at STP.

A2. 3.0 g x(1 mole/44 g) = 0.068 moles

0.068 moles x ( 22.4 liters/1 mole) = 1.5 liters

Q3. If the flask holds 200 mL of gas above the liquid, explain what happens to the extra gas.

A3. The CO₂ diffuses through the soap film. CO₂ diffuses rapidly because it is quite soluble in the film.

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-

diffusion, bubbles, soap films, gases, solubility, sublimation, dry ice