Expt 030 -- Liquid CO₂

A small sample of dry ice is placed in the bulb of a plastic transfer pipet. The stem is clamped shut. In a matter of seconds, the subliming chunks of dry ice begin to melt and then boil at the same time. A Boyle's Law-type micro gauge can be incorporated, and experimental values for CO₂'s triple point pressure can be determined.

Chemical Concepts

1. The triple point of a substance is the precise temperature and pressure that allows the solid, liquid and vapor states of that substance to all exist in equilibrium. At the triple point, subliming, melting and boiling all occur simultaneously. On the phase diagram, the triple point is where the solid-liquid, liquid-vapor, and solid-vapor equilibrium lines all intersect.
2. Liquid does not form below the triple point pressure of a substance. Solids sublime directly to vapor without forming liquid.
3. The pressure of a gas is inversely proportional to the volume of the gas. Boyle's Law: P₁V₁ = P₂V₂

Background

• A look at the partial phase diagram of CO₂ shows, as one would expect, that CO₂ is most stable in the gaseous state at normal room conditions -- approximately 1 atm and 20ºC. See below.

  

• The phase diagram also shows how solid CO₂ earned its nickname "dry ice," for if a sample of it is left out in the room, it does not melt the way ordinary ice does; it sublimes. That is, it undergoes a phase change directly from a solid to a gas, as represented by the dotted line. One should not assume, however, that liquid CO₂ does not exist, it simply is not stable at the relatively low pressure of 1 atm. To be able to observe liquid CO₂ then, one must be at or above CO₂'s triple point pressure of 5.1 atm (see diagram above).
• In this experiment, the pressure of the CO₂ is raised by sealing the container. As dry ice absorbs heat from the bulb's surroundings and sublimes, the trapped CO₂ gas causes a significant increase in pressure -- making the triple point attainable.
• This activity gives the you an opportunity to witness something rarely seen: the high-pressure melting of dry ice and the subsequent re-freezing of the liquid when the pressure is decreased.

Safety

• Wear goggles, apron and thermal gloves.
• Under increased pressure, the plastic bulb holding the dry ice does explode and/or rupture. Dry ice is so cold that it causes burns -- tissue damage. Handle powdered dry ice with extreme care. Use thermal gloves when filling the pipet with dry ice.
• Perform the experiment inside a plastic cup of water, if the bulb explodes, potential flying debris is contained by the water. Wear goggles and apron. USE A PLASTIC CUP, NOT A GLASS ONE. THE RUPTURING BULB COULD BREAK THE GLASS.

Procedure

1. Place 3-4 grape-sized chunks of dry ice on a newspaper, fold the newspaper over on top and tap with a hammer or the side of a pair of pliers to crush the dry ice into small rice-grain-sized pieces. Small pieces are better than powdered dry ice.
2. Place 2-3 small rice-grain-sized pieces of dry ice on the table and observe them until they have completely sublimed.
3. Fill the plastic cup with tap water to a depth of 4-5 cm.
4. Cut the tapered tip off the pipet.

   !!!Click here to See Movie.

5. Scoop some of the dry ice pieces into the stem, and then tilt it upward to slide the pieces down into the bulb. Continue scooping and tilting until the bulb is about half-filled with dry ice.

   !!!Click here to See Movie.

   Optional step:

   In the next step, two short steel rods, one placed on either side of the pipet stem, can help concentrate the squeeze of the pliers, making the job of clamping the stem shut possible for people with limited hand strength. Use a rubber band for holding the two rods together at one end. Place the clamp around the stem of the plastic pipet. Clamp the two strips tightly with a pliers to test.

   !!!Click here to See Movie.

6. Using a pair of pliers, clamp the opening of the stem securely shut so that no gas can escape, and, holding it by the pliers, immediately lower the pipet into the disposable plastic cup of water just enough to submerge the bulb.

   !!!Click here to See Movie. Click |> or <| to step the slides forward or back.

7. Observe from the side of the cup the behavior of the dry ice. As soon as the dry ice has melted, carefully loosen the grip on the pliers, still holding the bulb in the water, and observe the CO₂. Tighten the grip on the pliers again and observe. Repeat several times.

   !!!Click here to See Movie.

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   Record all changes that occur.

   Note: If you hold the pipet shut too long, the bulb does rupture, but the force, noise and potential shrapnel are all absorbed by the water. In exchange, a lively splash can occur. (This is the student's favorite part!) Sometimes, the force is great enough to split the side of the plastic cup. Have sponges and mops ready!

   !!!Click here to See Movie.

Boyle's Law-type micro gauge

1. Use a pulled thin stem plastic pipet to insert a drop of dark colored water near the open end of the micro gauge.

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2. Obtain a new pipet and repeat steps 2-5 above. .
3. Record the initial air volume in the gauge by reading to the inside edge of the water droplet. Slip the gauge open-end down into the pipet of C0₂. Support the gauge above the CO₂ by the thread held in place with the metal bars or the pliers.
4. Clamp down on the pipet to seal it shut. Make sure to clamp down on the supporting thread and not the gauge itself. Record the new gauge volume when the dry ice just starts to melt.

   !!!Click here to See Movie. The movie is accelerated by 5 times.

Questions

1. What did you observe when the dry ice sublimed? What did you observe when it melted?
2. How is the melting of dry ice different than the melting of ordinary ice? What ideas can you offer to explain these differences?
3. Treating the phase diagram as a "map", try to sketch the "path" taken by the CO₂ as it melted inside the closed bulb.

   

4. As you melt and re-freeze the CO₂ sample over and over, why did it eventually get used up? Can you think of ways to get more melting-freezing cycles out of a given dry ice sample?
5. What purpose(s) do you suppose the water in the cup serve(s)?

   Think carefully about the following three questions, develop and write down your best hypotheses and, if time permits, go back and test your hypotheses with the dry ice itself.

6. What might have happened if fewer pieces of dry ice (only 1/10 of the bulb, for example) had been placed inside the pipet bulb?
7. What might have happened if more pieces of dry ice (a full bulb, for example) had been placed inside the pipet bulb? (Consider specifically the amount of time the process would have required.)
8. What might have happened if the grip on the pliers had not been released once the dry ice melted? What if the pipet were extremely strong?
9. Use initial and final volume measurements of the air sample in the micro pressure gauge to calculate an experimental value for CO₂'s triple point pressure (the pressure in the bulb when the CO₂ starts melting).

Handout

Name ___________________________ Class _______

Teacher __________________________

BeckerDemos 030 Liquid CO₂

Handout Makeup

Name ___________________________ Class _______

Teacher __________________________

BeckerDemos 030 Liquid CO₂

Watch the movies. Carefully record observations.

Carefully record the measurements from the gauge in the movie to use for question 9.

Answer the questions.

Curriculum-

This experiment is ideal for discussions of phase changes and more specifically phase diagrams. This experiment relates to a range of phenomena -- refrigeration, evaporation, melting, and others.

Activity-

Laboratory or Demonstration

This activity works well as a laboratory or demonstration.

Safety-

• Wear goggles and apron.
• Frost bite can result from contact with dry ice. The teacher should crush the dry ice so the only contact students are likely to have is with the crushed particles.
• The pipet bulbs are under considerable pressure when the stems are sealed -- over 5 atmospheres at the triple point, for example. Explosions -- rapid failures -- do occur. THE PLASTIC CUP OF WATER IS A NECESSARY SAFETY FEATURE. Without it, very loud "POPS" can occur, possibly damaging hearing. Also, without the water, the plastic bulb can conceivably become very cold and rigid. Rather than tearing open, the way it does when kept supple under water, the bulb can rupture and possibly send out hard, high-speed fragments.
• When using the plastic cup, the primary hazard comes from the force of the splash. WEAR GOGGLES AT ALL TIMES during this activity.

Time-

Teacher Preparation: 5 minutes (plus time to obtain dry ice)

Class Time: 15 minutes

Materials-

• 4-5 g of dry ice, broken up into small rice-grain-sized pieces (See Lab Hints.)
• 1 transparent plastic cup
• 1 Jumbo plastic pipet with a 3-4 mm id stem cut to 5 cm
• tap water
• scissors
• pliers
• file clip (optional)]
• thermal gloves

Optional micro gauge (See Lab Hints)

• food coloring
• 1 thin stem pipet

Disposal-

Allow unused dry ice to sublime under the hood. Discard ruptured pipet bulbs in the appropriate plastic recycling container.

Lab Hints-

• Dispense the gauges. Set out a beaker of water with food coloring and 3 or 4 pulled thin stem pipets for sharing.

Construction of Optional gauge:

1. To make the pressure gauge, cut a 12 cm tip from a straight thin-stem pipet. Push a small droplet of hot glue into one end of the tip.

   !!!Click here to See Movie.

2. Place a string in the hot glue and add more hot glue to hold the string and seal tightly. Use a ruler to mark even increments on the tube. Use care locating the zero point where the glue begins. Cut the opposite end at an angle to make filling the gauge easier.
3. Pull a thin-stem pipet to make a long very narrow tip for filling the gauge. To do this, bend the tip around one finger and grasp tightly. Grasp the bulb and stem with the other hand and pull. The plastic stretches and narrows.

   !!!Click here to See Movie.

4. Cut the narrow portion with a scissors. Leave a long tip on the bulb to insert liquids into small tubes.

   !!!Click here to See Movie.

Dry Ice:

• Obtain dry ice from an ice house, a packing house, an ice cream shop, a grocery store, a hospital, among other sources. Dry ice can be manufactured from a CO₂ fire extinguisher by discharging the gas through a piece of cheese cloth. Recharging fire extinguishers is expensive, so this should be done as a last resort and only when other fire extinguishers remain available to deal with emergencies.
• Dry ice is provided in blocks. The easiest way to prepare crushed dry ice is to wrap a chunk (about 10 cm x 10 cm x 10 cm) in a strong towel or piece of canvas and twist the cloth creating a sort of ball with a handle. Smash the ball repeatedly on a hard surface -- such as a concrete floor or sidewalk. A hammer or the side of a pliers also can be used for crushing to the desired size, but wrapping the dry ice in a strong cloth is the key to success.
• Store the dry ice in a Styrofoam cooler. Keep thermal gloves handy to handle the dry ice. Wrap the crushed dry ice loosely in several layers of cloth or newspaper when setting is out for students.

Observations-

After the bulb has been clamped shut for some time, a point is reached where the bulb suddenly seems to frost over. This occurs just before melting. Heat transfer is more efficient at that time, and the very cold bulb becomes especially efficient at condensing moisture from the air. After a few more moments, a slurry is seen in the bulb. The remaining dry ice pieces stay on the bottom, for they are denser than the liquid CO₂. (It is important to point out that most substances behave like CO₂ and become less dense upon melting. Solid H₂O, with its open lattice structure caused by extensive H-bonding, is the exception to the rule.) Still later, the slurry clarifies to a colorless liquid. At this time, the pressure rises quickly and the bulb is in danger of rupturing. When the stem is released, the pressure falls drastically, a loud hissing sound is heard, the expanding gas cools (Joule Thomson effect -- the working basis for most refrigerators), and most of the remaining CO₂ (that does not escape) solidifies again. Small crystals of CO₂ with well-defined edges are observed. When this is clamped off again, the cycle repeats itself. The melting is much swifter and more pronounced the second time, since the powder is fluffy and contains more surface area for heat transfer to occur. The cycle can be repeated several times, but eventually the dry ice sample becomes so diminished that there is no longer enough to build up the necessary pressure, and so the sample simply sublimes away.

Answers-

Q1. What did you observe when the dry ice sublimed? What did you observe when it melted?

A1. The dry ice sublimes steadily and slowly. Depending on the humidity level in the room, clouds of vapor can condense around the dry ice and form a small cloud there. The dry ice melts rapidly when the pressure reaches the triple point.

Q2. How is the melting of dry ice different than the melting of ordinary ice? What ideas can you offer to explain these differences?

A2. Once the triple point pressure is reached, dry ice melts much more rapidly than ordinary ice. The energy required to melt water is much greater than the energy required to melt CO₂--i.e.. the heat of fusion of water is greater. Also, the remaining dry ice pieces stay on the bottom, because they are denser than the liquid CO₂. Ordinary ice floats in its liquid, because it is less dense.

Q3. Treating the phase diagram as a "map", try to sketch the "path" taken by the CO₂ as it melted inside the closed bulb.

A3.

As the temperature increases, the solid sublimes to vapor which increases the pressure, and the sample moves up the solid-vapor equilibrium line. When it reaches the triple point, the solid begins to melt and the liquid begins to boil. It hovers at that triple point for a while. Theoretically, with adequate mixing, the sample should remain at the triple point pressure and temperature until all the solid has melted, at which point the sample would begin to climb the liquid-vapor equilibrium line, and the pressure and temperature would again increase until the bulb ruptures. This would imply that there is no chance of the bulb rupturing as long as there is still some solid present. The fact that the bulb often does rupture before the solid is gone indicates that mixing is not adequate and that part of the sample is leaving the rest behind and starting to climb the liquid-vapor line and increase the pressure.

Q4. As you melt and re-freeze the CO₂ sample over and over, why does it eventually get used up? Can you think of ways to get more melting-freezing cycles out of a given dry ice sample?

A4. Each time that the pressure is released, CO₂ in the vapor state escapes from the bulb. If the pressure is only partially released (down to say, 4 atm, instead of all the way back down to 1 atm), then less CO₂ is wasted, and less time is required to melt the remaining sample again.

Q5. What purpose(s) do you suppose the water in the cup serve(s)?

A5. The water increases heat transfer to melt the dry ice faster. It also keeps the bulb supple and prevents condensation of water droplets or ice on the outside of the pipet, which might obscure the view. Even better, the water acts as a lens to provide a magnified view of the phenomenon. Finally, when the bulb does rupture, the water helps to muffle the sound and to catch any potential shrapnel.

Q6. What might have happened if less dry ice (only 1/10 of the bulb, for example) had been placed inside the pipet bulb?

A6. If the sample were too small, the pressure may not reach the triple point, and the entire sample sublimes and show no sign of melting at all.

Q7. What might have happened if more pieces of dry ice (a full bulb, for example) had been placed inside the pipet? (Consider specifically the amount of time the process would have required.)

A7. The first appearance of liquid would be more rapid because the pressure would rise to the triple point more rapidly. Melting the entire sample, however, might take longer.

Q8. What might have happened if the grip on the pliers had not been released once the dry ice melted? What if the pipet were extremely strong?

A8. The pressure would build up and the bulb would rupture. Then the pressure would drop instantly and the remaining sample would solidify immediately into a powder. If the bulb were strong enough not to rupture, the sample could climb the liquid-vapor equilibrium line all the way up to room temperature. The bulb would have to be able to withstand a pressure of more than 80 atm though. It could then be warmed a little further to observe the critical point where the liquid and the vapor phases become indistinguishable!

Q9. Use initial and final volume measurements of the air sample in the micro pressure gauge to calculate an experimental value for CO₂'s triple point pressure (the pressure in the bulb when the CO₂ starts melting).

A9. From the movie,

V₁ = 8.0 V₂ = 1.5 P₁ = 1 atm (assumed)

You readings will be slightly different.

P₁V₁ = P₂V₂

P₂ = 1 atm (8.0/1.5) = 5.3 atm

Key Words 1-

triple point, evaporation, vapor, liquid, phase, phase diagram, solid, sublimation, molecular solids, cooling on expansion, expansion