016 Shrinking Suds

Expt 016 -- Shrinking Suds

Liquid detergent is added to a graduated cylinder containing some dry ice subliming under water, and the foggy carbon dioxide bubbles that form spill out over the top. Once in contact with the air, however, these bubbles begin to shrink quite noticeably.

Chemical Concepts

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.
Different gases can have very different solubilities in a given solvent. For instance, CO₂ is considerably more soluble in water than is N₂.
For gases under similar conditions, densities are essentially proportional to molecular mass. The buoyancy of a gas bubble in air depends on the relative molar masses of air and of the gas.

Safety

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

Procedure

Fill the graduate roughly one-third of the way with warm water.
Drop in 3-4 nickel-sized chunks of dry ice. Allow the students to observe them for a while. Notice the rapid sublimation and the "fog" produced.
Add a small squirt (2-3 mL) of dish detergent.

!!!Click here to See Picture.
One soon observes hundreds of fog-filled bubbles quickly accumulating on top of the water. As new ones form, the bubbles on top get pushed upward, past the mouth of the container.
!!!Click here to See Movie.
Then a rather unusual phenomenon occurs: the bubbles on the outside layer begin to shrink. This is especially easy to observe by scooping out a hand-full of the large, golf-ball-sized bubbles.
!!!Click here to See Movie.
Watch a single bubble in the cluster as it shrinks in a matter of seconds to the size of a BB. Watching the entire cluster of bubbles is fascinating, but confusing. They appear almost alive as inner bubbles quickly push to the surface as the outer bubbles shrink.
Since these bubbles are safe to handle, give handfuls of freshly formed bubbles to groups of students to watch.

Ask students to watch one bubble.

!!!Click here to See Movie. Watch one bubble.

Repeat the experiment with the air. Observe the bubbles carefully. Use a compressed air line if available. Or make suds in a basin by moving fingers across the top. Scoop out the large bubbles. Avoid blowing through a straw because you complicate the experiment for students who think about the small amount of CO₂ introduced.

Questions

Describe the movement of CO₂ as the bubbles decrease in size
Why are some of the bubbles large after many bubbles shrink to nothing?
Why do the bubbles inside the graduated cylinder not shrink?
Why do the bubbles stop shrinking once they reach the size of BB's?

Handout Makeup

Name ___________________________ Class ________

Teacher __________________________

BeckerDemos 016 Shrinking Suds

Watch the movies. Answer the questions.

Curriculum-

Use this experiment when discussing gas solubilities, densities, and diffusion. This experiment is easy to use to make comparisons between gases. Use CO₂ and air to compare solubility properties. The Methane Mamba (BBExperiment 015) may be coupled with this demonstration to illustrate differences in densities and the relationship to molecular mass.

Activity-

Demonstration - Student or Teacher

This experiment makes a good demonstration for a teacher or for a student project. The shrinking bubbles are best for groups of 30 or less because it is difficult to see one bubble very far away.
The Shrinking Suds is part I in a Dry Ice/Soap Film Quartet of Activities.

Safety-

Use safety goggles and thermal gloves to handle dry ice.

Time-

Teacher Preparation: 5 minutes

Class Time: 10 minutes

Materials-

200 g of dry ice (one fist-sized chunk)
100 mL of 5% solution of dish detergent (100 mL -- In a bowl, mix approximately 5 mL detergent with 100 mL of water.) (Joy^® or Dawn^® work best)
2 tall narrow containers. (1 liter graduated cylinders work well, but a tennis ball container or cut-off soda bottle also works.)
thermal gloves for handling dry ice

Optional:

30 cm of rubber tubing for bubbling air

Disposal-

Allow dry ice to sublime.

Lab Hints-

This experiment works on days that are too dry to form large CO₂ bubbles like the Shrinking Ball and Crystal Ball.

Wait until you are ready to begin the demonstation to break the large chunk of dry ice into smaller pieces. This is most easily done by wrapping the large chunk in a piece of cloth or newspaper and tapping it gently with a hammer.

Observations-

The Methane Mamba (BBExperiment 015) can be compared with this experiment. The methane (CH₄ molar mass 16) is much less dense than air (primarily N₂-28 with some O₂-32) causing the bubbles to rise up. The CO₂ (molar mass 44) suds cascade down the side of the container because the bubbles are much denser than air.
The stable state for CO₂ at normal room conditions is the gaseous state. And since barometric pressure (1 atm) is well below CO₂'s triple point pressure (5.1 atm), the liquid state for CO₂ is not stable. Thus, solid CO₂ (dry ice) changes directly to a gas; that is, it sublimes.
Out in the air, it is hard to notice this sublimation, for the CO₂ gas produced is clear and colorless. When placed in water, however, this sublimation becomes much more obvious, for now the escaping CO₂ can be observed as large, rapidly evolving bubbles. Also, water serves as a better heat sink than air, and so the sublimation is substantially faster.
It is important to point out that the thick fog that you see is not the CO₂ gas; it 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 -- much like the fog you see when your warm, humid breath is exhaled into cold winter air.)
The foggy CO₂ bubbles tend to shrink when exposed to air simply because the CO₂ gas, being relatively soluble in water, can diffuse readily across the soap film membrane. With air on the outside of the bubble, the higher CO₂ concentration is obviously on the inside, and the net diffusion would thus be outward. For the same reason, N₂ (and O₂) from the air in the room would be diffusing into the bubble. This should counteract the shrinking caused by the exiting CO₂; but since CO₂'s solubility in water is dramatically greater than air's, the net effect is one of shrinking. The inward diffusion of air could explain, however, why the bubbles reach a certain minimum size of about 1-2 mm in diameter and then stabilize. This minute volume might represent the air that has diffused in while the CO₂ was diffusing out. CO₂ is about 80 times more soluble in water then nitrogen; the detergent may alter this ratio.
It is also worth noting that the CO₂ bubbles do not appear to shrink as they are rising up inside the container; for there, they are surrounded by other bubbles which presumably contain inside them an equally high concentration of CO₂. This is not to say that CO₂ diffusion between bubbles is not occurring. Rather, because the CO₂ concentrations are essentially the same on either side of the soap film, the diffusion rates into and out of the bubbles in the cylinder are more or less the same.

Answers-

Q1. Describe the movement of CO₂ as the bubbles decrease in size. A1. The CO₂ from both the bubble and the surrounding air dissolves in the soap film. Because the concentration inside the soap film (100%) is much greater than the concentration outside the film (less than 1% initially) more CO₂ dissolves from the inside. CO₂ diffuses evenly out of both sides of the film. The net result is CO₂ leaving the bubble.
Q2. Why are some of the bubbles large after many bubbles shrink to nearly nothing? A2. The bubbles which are surrounded by other CO₂ bubbles do not shrink because CO₂ is diffusing into the bubbles at the same rate it is diffusing out. The surrounding gas is nearly 100% CO₂. When the bubble is surrounded by air, the CO₂ diffuses out of the bubble faster than air diffuses into the bubble because CO₂ is more water soluble than air. Yet, as the outer bubbles shrink, they expose the larger inner bubbles, which in turn shrink. This keeps the bubbles in a constant squirmy motion as they shrink.
Q3. Why do the bubbles inside the graduated cylinder not shrink? A3. The bubbles inside the graduated cylinder do not shrink because they are surrounded by bubbles containing the same CO₂ concentration that they possess. This does not mean that there is no diffusion taking place; it means that CO₂ is diffusing into the bubbles just as quickly as it is diffusing out!
Q4. Why do the bubbles stop shrinking once they reach the size of BB's? A4. The bubble size seems to stabilize (stop decreasing) at about the size of a BB because essentially all the CO₂ has diffused out, and it has been replaced with a considerably smaller volume of air which diffused in during that time.

References-

Acknowledgments: David Brooks of University of Nebraska, Lincoln, was the first to show me the wonderful effects that can be achieved by incorporating soap films into dry ice demonstrations.

Key Words 1-

diffusion, dry ice, density