Expt 071 -- Triple Point Phase Transition for Carbon Dioxide

A small sample of dry ice is placed in the bulb of a plastic transfer pipet. The stem is clamped shut. Under these conditions the solid absorbs heat, the pressure grows, and the solid melts when the critical pressure is reached.

Background

• A substance sometimes is found in the solid, liquid, and gaseous states of matter. Water, for example, is well known to exit in all three phases. Sometimes a substance will have several solid phases -- each with a unique set of physical and chemical properties. There are several solid phases of water formed at very high pressures. Information about phases can be summarized in a diagram called a phase diagram.
• Suppose a container is available filled with only one substance -- say water -- with no other substances present (like no air). Also, suppose there is a way to control both the temperature and the pressure inside the vessel. (Imagine a very large hypodermic syringe where temperature is controlled by cooling or heating the surroundings, and pressure is controlled by pushing or pulling on the syringe plunger.) A graph or diagram constructed by controlling the pressure and temperature, waiting for equilibrium, and then observing to see what phases are present is shown below.

  

• At high temperatures and low pressures, only the gas phase exists. At low temperatures and high pressures, only the solid phase exists. The remarkable observation is that two phases coexist only on one of the lines. The curve labeled TA describes boiling. Only at the conditions of temperature and pressure described by that curve will liquid and vapor be seen boiling.
• The most interesting feature of the curve is the triple point -- the point with a combination of pressure and temperature such that all three phases may coexist. When a glass of ice water is covered with plastic, this represents coexistence of three phases, too. In this case, however, air is present. When air is absent, the vapor pressure of water at the triple point is just under 4 mm Hg. At pressures below the triple point, the liquid phase cannot exist as a stable phase.
• Phase diagrams have many features that can be studied. When the triple point of a substance occurs at a pressure above room pressure, then the liquid phase is never stable at room pressure. In materials with this property, evaporation takes place directly from the solid to the vapor, a process called sublimation. The pressure for the triple point of CO₂ is about 5 atmospheres.
• 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 -- a makes the triple point attainable. [When dry ice is left exposed in a room, it sublimes; the pressure of CO₂ never gets high enough to reach the triple point.

Safety

• Under increased pressure, the plastic bulb holding the dry ice will 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 and goggles when filling the pipet with dry ice.
• Perform the experiment inside a plastic bag so that, if the bulb explodes, flying debris is trapped. Wear goggles and apron.

Procedure

1. Use the stem of a cut-off transfer pipet with a wide stem to scoop solid dry ice powder into the bulb. Place about 1-1.5 mL of solid CO₂ in the bulb.

   !!!Click here to See Movie.

2. Place the bulb inside a zipper locking plastic bag.

   !!!Click here to See Movie.

3. Using a needle nose or other pliers, grasp the stem of the bulb until it is completely sealed. Gently agitate the solid while holding the pliers tightly to close the stem. If water condenses on the bulb making visibility inside the bulb low, wipe the bulb with a tissue. Note and record all changes that occur.

   !!!Click here to See Movie.

   !!!Click here to See Movie. This movie is 5 times normal speed.

4. When the solid has melted but before the bulb swells and ruptures, release the pliers. Note and record all changes that occur. Repeat.

   !!!Click here to See Movie.

Questions

1. Elemental iodine is a stable solid in a sealed jar at room temperature. When the jar is opened for a long time, the iodine sublimes and disappears. Predict the pressure of the triple point for iodine.
2. In cold climates -- like Fairbanks, Alaska during mid-winter -- fallen snow seems to disappear without melting. Explain this observation.

Handout Makeup

Name ___________________________ Class _______

Teacher __________________________

SmallScale 071 Triple Point Phase Transition for Carbon Dioxide

Watch the movies. Carefully record observations.

Answer the questions.

Curriculum-

This is discussed with phases and phase transitions. Changes of state fit with this material. This experiment relates to a range of phenomena -- refrigeration, evaporation, melting, and others.

Safety-

• Wear goggles and apron.
• Burns resulting from contact with dry ice are a hazard. The pipet bulbs are under considerable pressure when the stems are sealed -- over 5 atmospheres at the triple point, for example. Explosions -- rapid failures -- during which powdered dry ice is dispersed into the air do occur. Therefore, stress two kinds of precautions. First, the teacher should powder the dry ice so the only contact students are likely to have is with the powder. Next, place each bulb inside a plastic bag so that, if a failure occurs while under pressure, the dispersed solids are trapped inside that bag.
• Wear goggles and apron.

Time-

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

Class Time: 15 minutes

Materials-

• Dry ice.(See Lab Hints.)
• plastic bag
• gloves (thermal)
• bulb of transfer pipet with a 3-4 mm id stem cut to 5 cm length

Disposal-

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

Lab Hints-

• Obtain dry ice from an ice house, a packing house, some grocery stores, some hospitals, 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 powdered dry ice is to wrap a chunk (about 10 cm x 10 cm x 10 cm) in a strong towel of piece of canvas and twist the cloth creating a sort of ball with a handle. Then smash the ball repeatedly on a hard surface -- such as a concrete floor or sidewalk. A rubber mallet or other hammer also can be used, but wrapping the dry ice in a strong cloth is the key to success.
• Store the dry ice in a Styrofoam cooler. Keep gloves handy to handle the dry ice. Wrap the powdered dry ice set out for students loosely in several layers of cloth.

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. 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) resolidifies. Small crystals of CO₂ with well-defined edges are observed.

Answers-

Q1. Elemental iodine is a stable solid in a sealed jar at room temperature. When the jar is opened for a long time, the iodine sublimes and disappears. Predict the pressure of the triple point for iodine.

A1. The pressure for the triple point of I₂ must be higher than one atmosphere.

Q2. In cold climates -- like Fairbanks, Alaska during mid-winter -- fallen snow seems to disappear without melting. Explain this observation.

A2. The temperature is so low that the liquid state is never stable; water sublimes from the solid to the gaseous state.

Reference-

This experiment was first developed by Robert Becker of Kirkwood, Missouri.

Key Words 1-

critical pressure, critical temperature, evaporation, gas, liquid, phase, phase diagram, solid, sublimation, molecular solids, cooling on expansion

Elements-

C