Expt 019 -- CO₂ Leaky Faucet

Although you have probably seen a leaky faucet, you have probably never seen one that drips carbon dioxide gas! Foggy CO₂ gas -- from dry ice subliming under water -- is passed through a faucet-like series of pipes, the mouth of which has a soap film across it. The dense CO₂ bubbles that form, pinch off and fall rapidly downward, resembling a huge leaky faucet!

Chemical Concepts

1. Dry ice is the solid form of carbon dioxide. At normal atmospheric pressure, it tends to sublime (go directly from a solid to a gas) rather than melt.
2. Whether an object floats or sinks in a fluid depends on whether that object's density is less than or greater than the density of the fluid.
3. Different gases can have very different solubilities in a given solvent. For instance, CO₂ is considerably more soluble in water than is N₂.

Procedure

1. Half-fill the bottle with warm water. Drop in the dry ice, and cover the opening securely. Fog cascades from the pipe if all connections are tight. Uncover the opening.

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2. Hold the cup of soap solution up to the mouth of the faucet to wet the paper towel and to establish a soap film across the opening.

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3. Cover the opening again. The first bubble appears clear until the CO₂ starts to enter the bubble.

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   Observe as a procession of misty CO₂ bubbles form and fall like huge, pearly water drops from a giant leaky faucet!

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   Note the vapor trail when the bubbles pop.

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4. You can try bouncing and/or catching the falling bubbles on a dry, recently washed T-shirt or piece of cloth, or on a large flat soap film - like a trampoline. Likewise, you can try catching them with a pre-wetted hand. Either way, the bubbles gradually shrink as the CO₂ diffuses out of them!

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5. You can try igniting the falling bubbles to show that, unlike methane or hydrogen bubbles, carbon dioxide bubbles do not light; in fact, they do extinguish a flame! Thus, not only do different gases vary in their physical properties such as density, they also vary in their chemical properties such as flammability. Point out to the students, however, that these two properties are not in any way linked. That is, there are some low density gases such as helium that do not burn, and plenty of high density gases such as propane and ether that do burn!

Safety

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

Questions

1. Why do the bubbles fall from the faucet?
2. If CO₂ is a clear, colorless gas, why are the CO₂ bubbles white and opaque?
3. Why is the first bubble that forms big, and then all the other bubbles small?
4. What parameters of the experiment could have been changed to make the bubbles fall more frequently.
5. What physical properties of CO₂ are illustrated by this demonstration?

Handout Makeup

Name ___________________________ Class _______

Teacher __________________________

BeckerDemos 019 CO₂ Leaky Faucet

Watch the movies. Answer the questions.

Curriculum-

This activity ties in well with a discussion of density -- especially the comparative densities of various gases. These "lead bubbles" would be nicely contrasted to ones filled with helium or methane or air.

Activity-

Demonstration - Teacher Only

The CO₂ Leaky Faucet is part IV in a Dry Ice/Soap Film Quartet of Activities. It works best as a demonstration.

Safety-

Dry ice is extremely cold and can cause frost bite. Thermal gloves are recommended whenever handling dry ice.

Time-

Teacher Preparation: 10 minutes (20 minutes additional for construction -- first time only)

Class Time: 5-10 minutes

Materials-

• 2-3 chunks of dry ice, 100-120 g each
• 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 clear, 2-L plastic soda bottle
• 3 lengths of 3/4" PVC pipe (60", 15" and 4")
• 2 elbow joint connectors for 3/4" PVC pipe
• 1 6-cm length of plastic tubing (1" ID)
• a paper towel
• rubber band
• tall, sturdy ring stand with two test tube clamps
• plastic cup

Disposal-

Allow the dry ice to sublime.

Lab Hints-

1. Prepare the water reservoir by one of the following:

   1. A. Cut off and discard the top 1/5 of one 2-L soda bottle, and cut off and discard the bottom 2/3 of a second 2-L bottle. The bottom 4/5 of the first bottle should have a hole 8-10 cm across and should fit snugly up into the top 1/3 of the second bottle as shown in the following Figure.

      

      or

   2. A hole can be cut in the side of a 2-L soda bottle, and the top half of a plastic (peanut butter) jar can be cut off, wedged in, and caulked into place. Then, instead of having to wedge the two parts together, you can simply screw the jar's lid on. The initial construction takes longer, but the screw lid is easier to use if you repeat the demonstration regularly.

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2. Use the three pipes and two elbow joints to construct a large faucet head (like an up-side down "J"). Then roll up the paper towel and wrap it around the mouth of the faucet. Use the rubber band to hold it in place there.

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3. Use a short length of plastic tubing to connect the screw top of the reservoir to the PVC pipe.

   !!!Click here to See Picture.

4. If you are using the two piece reservoir, place a 5 -10 cm support for the reservoir on the desk top. To fill the reservoir, remove the support, and add material to the bottom. Then, reinsert the bottom into the top, and slide the support under it again.

5. Use the ring stand and clamps to secure the faucet head over the bottle, so that the faucet is facing the students and extending out over the edge of the demonstration bench.

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 bottle is CO₂. 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 moisture comes in contact with the cold subliming CO₂ -- like the fog you see when your warm, humid breath is exhaled into cold winter air).
• The misty bubbles fall from the faucet remarkably fast due to CO₂'s relatively high density. This is not to say that Galileo was wrong -- that denser bodies have a greater gravitational acceleration. Remember that you are not working in the vacuum postulated in beginning Physics problems dealing with dense solids. When comparing properties of gas bubbles, you must consider the buoyancy from air. The CO₂ bubbles fall quickly compared to air bubbles. Remember that hydrogen or helium bubbles rise.

Answers-

Q1. Why do the bubbles fall from the faucet?

A1. The bubble material itself (the soap film) is more dense than air. Carbon dioxide is more dense than air. The condensed water is more dense than air. So, all parts of the bubble system are more dense than air, so the bubble sinks in air.

Q2. If CO₂ is a clear, colorless gas, why are the CO₂ bubbles white and opaque?

A2. Small droplets of water form when the cold CO₂ causes gaseous H₂O to condense to a liquid. As in steam and fog, these droplets are small enough to stay suspended but large enough to reflect light. Thus they make the bubbles opaque.

Q3. Why is the first bubble that forms big, and then all the other bubbles small?

A3. The first bubble begins to fill with air -- air that is pushed out of the faucet head by the advancing CO₂. Then the CO₂ starts streaming into the bubble, and a mixture of air and CO₂ forms. When the bubble reaches the point where its weight is greater than the adhesive forces holding the soap film to the faucet, the bubbles stretches downward, the film pinches off, the bubbles breaks away, and a new bubble begins to form. The second bubble and all the rest that form thereafter fill with essentially pure CO₂, which is more dense than the air-CO₂ mixture. Thus they all reach that break-away weight at a smaller size than the first bubble did.

Q4. What parameters of the experiment could have been changed to make the bubbles fall more frequently.

A4. If more dry ice had been used, there would be more surface area, a greater sublimation rate and therefore a greater frequency of bubble formation. Crushing the dry ice into smaller pieces would have a similar effect. Warmer water would also increase the bubbling frequency.

Q5. What physical properties of CO₂ are illustrated by this demonstration?

A5. The fact that the dry ice sublimes rather than melting illustrates that CO₂'s triple point pressure is higher than normal atmospheric pressure. The speed with which the bubbles fall (compared to similar-sized bubbles of air) illustrates CO₂'s high density. The shrinking of the bubbles illustrates CO₂'s high rate of diffusion through the soapy water, a result of its high solubility in water.

Reference-

Acknowledgements:

This is a variation on the time-honored blowing of hydrogen, helium or methane bubbles, only with a gas that is denser than air. It is also reminiscent of the lead balloon (balloon filled with xenon or sulfur hexafluoride) that falls at an amazing rate.

Further Reading:

• Soap-bubbles, Their Colours and the Forces Which Mold Them. C. V. Boys. Dover Publications, Inc., 1959.
• Tom Noddy's Bubble Magic. Tom Noddy. Running Press, 1988.

Key Words 1-

gases, density